I assume it's the Open Source version you're using as you've upgraded fro beta3? It shouldn't make any difference whether you've upgraded or just done a clean install, it should be enabled. The IM does work in RC2, I use it.
After a cursory glance at
bugzilla I can't see anything relating to your description and the hosted demo is pretty much the same as the released RC2. I'd suggest filing a bug report, that's why it's still beta and we need these to fix any problems.