First of all, check if you have JBoss Tools installed in your Eclipse IDE. You can find them here.
By default JBoss AS Adapter uses .metadata/.plugins/org.jboss.ide.eclipse.as.core/JBoss-AS-5.1.0.GA/deploy folder to deploy your project and then register this temp folder in JBoss. In other words, if you are starting JBoss AS from Eclipse, you have to deploy your project to this directory.
You can try this (taken from JBoss community site), helped me too:
Server configuration (in Servers view in your Eclipse IDE)
Deployment > "Use the JBoss Deploy folder"
Save Server configuration
Select your project on server and make a "Ful Publish" using project context menu
Start server
If you have everything configured correctly, the "add and remove" method from the previous answer should be working, too...
Let me know, if you got it working.