A few people seem to be having this problem. I don't run GameEx on my 64-bit machine (it's a dev machine) but I just tested it for you and GameEx seems to run Mame64 fine.
This is probably a long shot but have you tried all the video options in mame.ini ie. gdi, ddraw and d3d?
Let me clarify, mame runs beatifully on my rig, the problem is the front end.
Mamewah I cannot get to even fire up.
MaLa, when I point it to the 64 bit mame version, runs, but is unplayable. The speed is about half of what it should be, the keystrokes are completely unresponsive. Basically I am seeing the same results as posted here -
http://forum.arcadecontrols.com/index.php?topic=77163.0I tried Maximus Arcade, and it seems to work correctly, navigating the lists however was horrendously slow...
I tried to install GameEx last night, and it pointed me to the dotnetfx.exe, when it was done downloading, it errored out saying it could not install on a 64 bit OS, and the GameEx install bombed out completely. I thought about grabbing the NetFX64.exe and installing that, then seeing if GameEx can go through the install.
Did you install a fresh copy of GameEx or just copy over your 32bit files?