i have installed and run mechjeb once and it worked fine then the next time i run KSP mechjeb isnt working. i have tryed installing again and still it isnt working, then i tryed deleting the game and restalling and still not working then i got 2.0.7 and tryed that one and that one didnt work then i got 2.0.7 dev and that didnt work

The fact that it works for everyone else suggests the problem is with your system, not with MechJeb.

Assuming you're on Windows, are you running KSP as a user who has read/write access to your PluginData directory and everything within it?

i have set administrator rights and made sure all folders isnt writing protected and still it isnt working

I hate it when people do this, ask a question and then close it when there is no working answer or they don't say the answer that is working.

He probably just clicked "close and comment" by mistake. Even people experienced with github do that from time to time. :)

Poor bug reporting like "It doesn't work" is the biggest problem. People really should be referred to guidelines like http://www.chiark.greenend.org.uk/~sgtatham/bugs.html when that's all they can say to describe what's happening.