Well,I downloaded MechJeb2 in this forum last weekend.Yesterday I wanted to try it,but when I opened KSP, it prompted me that the game needed to be updated.So I updated it.After it finished,I opened it again and began to look for MechJeb part.Later I found that I couldn't find it,and my flag turned to white.I deleted it from my computer and reinstalled it in a folder.I opened my game again and I still couldn't find it.
I didn't use ckan,and I updated my game from Steam.I didn't delete anything before installing.Anyway,thanks for your advice!