Ok, here's what I did.
Haven't restarted my computer after disabeling the driver signature enforcement. The battery was out of the phone, I put it back into the phone, pressed and hold u-key while connecting the usb. Still holding the u-key for about 10-15 sec. Nothing more happened though, and a error message popped up "Device driver software was not successfully installed" and the usb connection-sound made the connected and then the disconnected-sound.
The usb-icon didn't show on the N900 either.

Isn't this the just the right way to do it? Can't really see what I did wrong here, but something is just not working. Tried several usb-ports too.