The plugin loads fine for me, I have the part and can attach it, but when I do no GUI tab appears. I'm running KSP 0.21 and have tried the main MechJeb 2.0.9 release, the 2.0.9.0-70 relase, 2.0.9.0-71 and 2.0.9.0-72.

The only clue is this appearing in the debug log:

[LOG 02:57:37.078] ------------------- initializing editor mode... ------------------
[LOG 02:57:37.078] editor started
[LOG 02:57:37.125] Untitled Space Craft - Untitled Space Craft
[ERR 02:58:46.140] MechJeb caught exception in core OnLoad: System.Reflection.ReflectionTypeLoadException: The classes in the module cannot be loaded.
  at (wrapper managed-to-native) System.Reflection.Assembly:GetTypes (bool)
  at System.Reflection.Assembly.GetTypes () [0x00000] in :0 
  at MuMech.MechJebCore.m__19 (System.Reflection.Assembly ass) [0x00000] in :0 
  at System.Linq.Enumerable+c__Iterator14`3[System.Reflection.Assembly,System.Type,<>__AnonType0`2[System.Reflection.Assembly,System.Type]].MoveNext () [0x00000] in :0 
  at System.Linq.Enumerable+c__Iterator1D`1[<>__AnonType0`2[System.Reflection.Assembly,System.Type]].MoveNext () [0x00000] in :0 
  at System.Linq.Enumerable+c__Iterator10`2[<>__AnonType0`2[System.Reflection.Assembly,System.Type],System.Type].MoveNext () [0x00000] in :0 
  at System.Collections.Generic.List`1[System.Type].AddEnumerable (IEnumerable`1 enumerable) [0x00000] in :0 
  at System.Collections.Generic.List`1[System.Type]..ctor (IEnumerable`1 collection) [0x00000] in :0 
  at System.Linq.Enumerable.ToList[Type] (IEnumerable`1 source) [0x00000] in :0 
  at MuMech.MechJebCore.LoadComputerModules () [0x00000] in :0 
  at MuMech.MechJebCore.OnLoad (.ConfigNode sfsNode) [0x00000] in :0 
[LOG 02:58:48.906] mumech.MJ2.AR202(Clone) added to ship - part count: 2
[LOG 02:58:48.906] stage count is: 0
[ERR 02:58:48.953] MechJeb caught exception in core OnLoad: System.Reflection.ReflectionTypeLoadException: The classes in the module cannot be loaded.
  at (wrapper managed-to-native) System.Reflection.Assembly:GetTypes (bool)
  at System.Reflection.Assembly.GetTypes () [0x00000] in :0 
  at MuMech.MechJebCore.m__19 (System.Reflection.Assembly ass) [0x00000] in :0 
  at System.Linq.Enumerable+c__Iterator14`3[System.Reflection.Assembly,System.Type,<>__AnonType0`2[System.Reflection.Assembly,System.Type]].MoveNext () [0x00000] in :0 
  at System.Linq.Enumerable+c__Iterator1D`1[<>__AnonType0`2[System.Reflection.Assembly,System.Type]].MoveNext () [0x00000] in :0 
  at System.Linq.Enumerable+c__Iterator10`2[<>__AnonType0`2[System.Reflection.Assembly,System.Type],System.Type].MoveNext () [0x00000] in :0 
  at System.Collections.Generic.List`1[System.Type].AddEnumerable (IEnumerable`1 enumerable) [0x00000] in :0 
  at System.Collections.Generic.List`1[System.Type]..ctor (IEnumerable`1 collection) [0x00000] in :0 
  at System.Linq.Enumerable.ToList[Type] (IEnumerable`1 source) [0x00000] in :0 
  at MuMech.MechJebCore.LoadComputerModules () [0x00000] in :0 
  at MuMech.MechJebCore.OnLoad (.ConfigNode sfsNode) [0x00000] in :0 
[LOG 03:00:03.281] Game State Saved as persistent
[WRN 03:00:03.296] [HighLogic]: =========================== Scene Change : From EDITOR to SPACECENTER =====================

w00t! Thanks folks!

it only took 2 years ;)

Better than never being done. I'll take it! :)

Tried with KSP 0.21.1 as well.

I eventually tracked this down to the Hexcans mod at http://kerbalspaceport.com/hexcans-standardized-resource-canisters-2/

This issue can be closed :)

Confirmed, once I removed this HexCan addon, MechJeb returned to function normally.