I had a lot of trouble finding documentation for this. If you want to make a call on the main thread in RubyCocoa, the format is something like:

performSelectorOnMainThread_withObject_waitUntilDone('method_name:', object, true)

So two things that took me a while:

1. method_name has a trailing colon.
2. The object the has the method you want to call should be the second argument. I got it in my head it should be the first for some reason.