In version 23.5, MechJeb stopped working completely. It's like it isn't installed whatsoever.

Same here.

The assembly will need to be updated. Looks like there're a few new things to implement. I'm working on a PR.

As I said in an other closed ticket there is a version with a fix that was available in the hour following 0.23.5 release. You can get it here : http://jenkins.mumech.com/job/MechJeb2/
Some bug are still present.

Still doesn't work after http://jenkins.mumech.com/job/MechJeb2/

Tried to install mechjeb 2.2, current build from http://kerbalspaceport.com/mechjeb/ also tried the one which was build today from the jenkins. In Both cases i can see that Kerbal Space Industries is loading the mod, but it just wont show up in the ui.

So i bought the game from steam, downloaded it, started it. Now i copied MechJeb2 to /ksp/GameData/MechanicalJeb2

when i tired to use the different build i restarted the game, in between i removed the previous version and copied the newer one. But the UI just wont show up ingame. do i have to set up something?

oh yeah, sorry my bad. The menu did not show up because i did not attach any mech jeb parts onto the ship so it couldnt take control. didnt know, thanks tho :)

Just to be sure you did put the part on your ship ?
If yes, is your game installed in Program file of the like ? Windows protection and some AV are over zealous with mods.