Creating a WSAS 3.1.x Instance is easy. Switch the IDE to the JavaEE perspective
You will see that there is the Server View available (normally in the bottom)
Right Click on the view New -> Server
Select WSO2 from the tree and under that select WSO2 WSAS 3.1.x and Click Next
Specify the WSAS home location and Click Next
Specify the http/https and debug ports you wish to use and click Next.
If there are any dynamic project you wish to add to the server you may add from this page and click Finish. Now you have a server instance.
To Start the server instance, right click on the server item in the Server view and select Start to start the server.