If you have an outlet to the window, call [theWindow setAcceptsMouseMovedEvents:YES]; after loading the nib. If this is in the main nib, you can do it in applicationDidFinishLaunching. If this controlled by an NSDocument subclass, do it in -windowControllerDidLoad:.
Alternatively, you could implement -awakeFromNib in your view class:
-(void)awakeFromNib
{
[[self window] setAcceptsMouseMovedEvents:YES];
}