i don't know what the cause is, but no GUI window or tab is showing up for Mechjeb and the debug menu is giving error messages for it at a very fast pace.

I did further research, and it appears that this may be caused by an incompatible version of the toolbar mod: #335

I was able to fix by upgrading to Toolbar 1.7.1. I'm not sure what version I was using before. You might try the same: http://forum.kerbalspaceprogram.com/threads/60863

This happens to me as well