I installed M&M9 on Win 7 64bit, but it's not working at all. I've tried running it in compatibility modes, and it's not working either.

It comes up with the launcher, then I get a maximised window of blackness followed by a "MMXI Has Stopped Responding" or something along those lines.

Drat :(

It works fine on my setup. Do you have Direct X 9.0 C installed? DX9, when installed should solve your problem. Direct X 1-9.0 C by default needs to be installed to properly run most older titles. I hope this solves your problem. The DX9 compatilbility problem started with Windows Vista, Windows 7 and Windows 8.

Edit: I have Windows 7 64 Pro.

Yes, I do. I've tried both installers from the Microsoft site (one dated '08, I think, the other '10 or '11) and neither actually installed as I have a newer version on my PC. I keep my Win 7 installation as up to date as I can.

The game still doesn't load.

I thought that would solve your particular problem. You should open a support ticket, I hope they can better direct you than I did. Good luck.

I've managed to fix it.

I followed the steps of yzx from this Microsoft support question and by altering that one registry code I've got the game working.