How do you add a ruby file to an existing project in Eclipse 3.6 using the Dynamic Languages Toolkit (DLTK)?

Question

I've installed the Dynamic Languages Toolkit (DLTK) in Eclipse 3.6. But I can't figure out how to create a Ruby file in an existing Java project.

Doing the obvious thing (New -> Empty Ruby Script) just gives the error message "Source folder is not a correct project". Do I have to modify the project in some way to tell DLTK that it's OK to add Ruby files to it?

(Adding the file outside of Eclipse does work.)