Currently Mogre only works with Ogre 1.7.
Some Mogre users tried to update the autowrapper to work with the changes of Ogre 1.8, did some steps, but couldn't solve all needed points.
So Ogre 1.8 and 1.9 aren't supported by Mogre yet.
We don't know when newer versions are supported, because updating the autowrapper process needs time (for the developers) and special knowledge about the techniques (C++/CLI, Doxygen, etc.).
Someone has allready compiled mogre with ogre 1.9 ? and if yes can i do it just by changing the repository in mogrebuilder ?
When you try it, you will see different errors.
I'm sorry that I can't give you a better answer.
Mogre 1.7 works well, but if you need newer features of Ogre, you have to (try to) update it yourself or use Ogre with C++.
If you want to stay with C#, possibly Axiom
is an alternative to Mogre. It's a C# port of Ogre, but doesn't support all new Ogre features.
Compiling Mogre against x64 is possible by the MogreBuilder tool.
Use the command-line option -x64
Keep in mind: When you use Mogre with 64 bit, all add-ons have to be combiled against 64 bit, too.
One user made some experiments with 64 bit Mogre, but couldn't see a huge performance benifit.
So I suggest to use x86 builds, because then you avoid trouble with add-ons. Most of them are precompiled only for 32 bit.