someon tell me why rpg builder 3D won't start.
It always says there is a problem.

Okay first of all Introductions isn't the place for a topic like this, moved it to the General Game Engines Discussion section.
Secondly before anyone would be able to help you , if they can, then you'd need to actually say what the problem is and what you've tried.
eg. Running as Admin is normally a common solution.