I originally connected with the SA account and the password we setup for it during install. I then created another SQL account to connect to from then on.
Not sure what it would be for your install. There's a way to reset the SA password but I think that would take down your LM install. There's probably a config file where this is held but I'm not sure where that is. If I had our dev LM instance still I would try to find it but I've rebuilt it with VCD.