Yeah that's always the fun ones.
Well there are a couple of ways.
1. The tags are in a jar or a DTD, and putting that in the classpath works.
2. There is also a place to add tags that are valid, but you type them in yourself. So if there are lots of tags, then you type a lot into the settings area.
I am going to start up IntelliJ to see if I can find that setting. I don't recall exactly where it is. And it might just be for javadoc tags, like those you use in XDoclet.
Mark