Basically you need to know how to configure and deploy applications to Websphere Application Server.
You can look at their redbooks
http://www.redbooks.ibm.com/. You can also get a Websphere trial download
http://www.ibm.com/developerworks/websphere/downloads/ to familiarize yourself.
IBM has the development tools known as RAD (Rapid Application Developer) or RSA (Rapid Software Architect, which are basically an extension of eclipse with integrated Websphere Application Server and other bells and whistles like EJB support, etc.
http://www.ibm.com/developerworks/downloads/r/rad/ Having said, lack of Websphere experience should not be the detrmining factor at the interview. It should be based on your JEE knowledge, skills, and experience. Once you know a particular application server, you can learn another one relatively quickly. Some organizations, do have a separate middleware team that is responsible for configuring and deploying to application servers.
Good luck.