I have a big problem too! After installing the latest Toolbar plugin all windows disappeared (Scansat, TAC Fuel Balancer, Mechjeb and more). First i think it was because of the toolbar. After several ways of cleaning i decided to reinstall my Windows 8 because my computer need it anyway. Now with a absolutely clean harddisc i installed KSP again and Mechjeb too but the Mechjeb menu is still disappeared! I tested a lot of things to get it work but nothing helped... The parts are there but no menu. I found some lines in the output log which are curious: Failed to load assembly C:\Program Files (x86)\Kerbal Space Program\GameData\MechJeb2\Plugins\MechJeb2.dll: System.UnauthorizedAccessException: Access to the path "C:\Program Files (x86)\Kerbal Space Program\GameData\MechJeb2\Plugins\MechJeb2.dll" is denied. at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean anonymous, FileOptions options) [0x00000] in <filename unknown>:0 at System.IO.FileStream..ctor (System.String path, FileMode mode) [0x00000] in <filename unknown>:0 at AssemblyLoader.LoadExternalAssembly (System.String file) [0x00000] in <filename unknown>:0 Hope somebody can help and sorry for my english!