I use MechJeb2 2.9.1.0 and KSP 1.8.1.2694 The problems that is when i do a bi-impulsive (Hohmann) transfer to target (the mun in the planner that do a direct flight to the mun and not an apoapsis or periapsis so i can't orbit around the mun. Can't you help me ?

There's no function to select the periapsis distance. Implementing that correctly will take months of work, it isn't simple. For now you can either manually adjust it with the node editor, or else use the "fine tune closest approach to target" (which is also somewhat inaccurate, because the problem is hard).

dup of #1060 i think (or one of the other epics)