I seem to be having the same problem as several folks here. MechJeb 2 simply isn't working. I have KSP via Steam and have completely deleted KSP and re-installed .20 a couple of times. I have no other mods running, and it doesn't matter if I install it the new way or the old way... nothing shows up in the game. I'm on Windows 7. When I first ran .20 after the update, the older version of MechJeb was there, but I couldn't use or select any of the parts and the textures were not correct. I was hoping MechJeb 2 would fix that, and it sounds like it's supposed to, but it just... doesn't show up in the parts. It does, however, briefly flash on the loading screen.