New to Adobe Atmosphere?
To view the 3D Models, you'll need the Adobe Atmosphere 3D player
And this is not one of those nefarious "scumware" plugins that installs advertisements or spyware into your computer.
Just click where it says Atmosphere Player when you get to Adobe's Atmosphere page.
The player itself is free, don't worry about that $399 price farther up the page - that's for the authoring tools to do own 3D modeling (a 30 day trial copy of that is free).
This is what the Adobe Atmosphere download page looks like:

Next you'll be taken to the actual download page, with instructions and a blurry 2D picture of an Adobe Atmosphere 3D scene.
Read the instructions, then click on the scene picture to download the player. When the download and install are done, the picture will clear. Then you can enter the 3D modeled scene (kind of like Alice stepping through the mirror to enter Wonderland ).
Just click on the cleared picture first, then use your keyboard arrow keys to enter and move around in your first Adobe Atmosphere world. And don't forget to turn on your sound.
This fun scene is a working model of the solar system. Try turning off Gravity in the player toolbar, then use your keyboard's Shift+Up arrow keys to fly up to the planets!

When you install the player, you may see a safety certificate from the Viewpoint Corporation. Viewpoint is one of Adobe's 3D technology partners and their software is included in the player.
Once your Adobe Atmosphere player is installed, you can just click on any Atmosphere 2D image on a Web page anywhere, in order to enter that 3D scene. Nowadays, almost all Atmosphere scenes are embedded inside web pages, just like the one above is.
2. To move, jump or fly inside an Atmosphere scene, I prefer to use the keyboard keys, rather than the mouse. It's a lot easier than it might look from this table. To fly, turn off "Gravity" (on the toolbar at the bottom of the player).
To pass through solid objects, turn off "Collide". Be sure "Collide" is on whenever "Gravity" is, or you will sink right through the ground and fall forever! But there is an easy way to reload the scene to save yourself.
One thing you may find a bit frustrating at first, is that if you leave an Atmosphere scene to adjust the player or do something in Windows, you must always click on the scene again before you can reenter it.

3. To learn to customize the player, or give your Avatar a nickname other visitors in a scene can see and so forth, it's a good idea to visit an Adobe Atmosphere "Training World" for a bit of practice. This world is no longer supported by Adobe, and so it's a bit dated. For example it doesn't mention right-clicking on a scene to get more menus.
4. If you would like to learn more about Adobe Atmosphere, or become a world builder yourself, please visit AtmosphereCentral, a web site for professional Atmosphere developers, or take part in the Adobe Atmosphere forum, where Atmosphere artists, hobbyists, novices, pros and everyone in between hangs out. Be sure to look at the bottom of the forum posts (you will need to use the "More" button) for the folder of "The Village" section of the forum. That's where the old-timers and longtime beta testers are often found. And my own resources page has a lot of links for you.
5. About inworld etiquette
Atmosphere is quite new and there are not so many scenes ("worlds") yet. Nor are there many visitors yet. So don't be supervised if many Atmosphere worlds you visit are empty.
Of course, if some of your friends (either the real world or online kind) have the Atmosphere player, you can all meet in an Atmosphere scene and go exploring together. And if you meet someone inworld, don't be shy, ask them if they know of any nice scenes they can give you "bookmarks" to. Or they might like to explore with you. Other ideas for finding occupied Adobe Atmosphere worlds are below.
Remember that people in Adobe Atmosphere scenes are represented by their Avatars. "Behind" each avatar is a person watching their PC monitor. Give them some respect. Don't run through them or get right in their face to see the details of their costume. Or at least say something like "Hey, nice AV, let me get a closer look." Just as in the real world, people like to preserve a bit of personal space around them. An Atmosphere scene is a public place, so put on the kind of "best behavior" you naturally would for a public place in the real world.
Really fancy and ultra-realistic avatars can take a lot of computer power to display and animate, possibly slowing down the whole scene for everyone. Until you know what you are doing, stick with the default avatars that come with the player. They are designed with scene speed in mind.
There are hundreds of Avatars available, in every conceivable form. Obviously, it is not polite to wear a naked human Avatar. But all kinds of other fantasy characters are common, especially around the Western Halloween holiday. In fact, some Adobe Atmosphere builders specialize in making avatars, rather than Atmosphere scenes.
Remember that Atmosphere scenes are international. You will often meet people inworld whose first language is not English. They may not understand your colloquial jokes, or may have difficulty following what you are saying. Go slow and help them along. And remember all the different time zones people are coming from, if you want to setup an inworld meeting. It helps to download a free world clock program.
6. More tips for beginners
Inworld slang and jargon. Communication is still by typing at a keyboard, so most of these items are meant to make typing faster and easier:
jimcoe(at)mindspring(dot)com