To open a world in Alice you must of course save your project. Save it to the Desktop or someplace easy to get to. Then to open go to File>Open world>and select the project with the file extension .a2w. To post a world on the forums select the "Manage attachments button" then go to upload and again select the appropriate file.
"Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe."-Albert Einstein
The forum does not yet allow uploading a3p files yet. As the program is still in beta. However, you could simply save your Alice 3 Project in a zip file and upload it as a zip file.
My Favorite Resources:
Uploading Images - http://tinypic.com/