Creating a new project on Linux, a build directory was not generated and so when I went to the "test in emulator" tab, nothing showed up. I don't remember what I did to fix this now, but I think after creating an empty build/android directory, it added the files I needed.
Anyway, I moved the project to a Mac later on to add iPhone support, and again the emulator tab was empty. This time simply creating an iphone directory was not enough. I had to create another project on the Mac, copy over its build directory, rename the files, search and replace in the files, and I was finally able to build.
Edit: sorry, that should read "run emulator"