I ran into that sort of problem too. Can't give you a full answer but,
First, have you read the post on the differences between the original JForum and the bugfix version on Google code? Hunt it down. It helps for those answering to know which code you are referring to. Yes, especially for this.
If you are using the bugfix version, I put a very full post on this forum about how to hack out the operative code from the bugfix version into a .war which can be imported into Eclipse, by using a precompiled version to effectively pre-configure Eclipse. It sounds hacky, but is efficient,and has few steps,
https://coderanch.com/t/623839/jforum/Import-Google-JForum-source-Eclipse
If you're taking another route, I can't offer so much help. Bear in mind, you will need the infrastructure that a 'Dynamic Web Project' offers. There may be a way to pre-configure the project using
Maven, but I didn't take that route.