I got one night at the Signature thru MyVegas. I know that there are three separate buildings. Does anyone know where the check desk is?
each tower has it's own check in desk. Guards will have your reservation and direct you to the tower. Most likely tower 3 if you booked through MyVegas or MGM direct.
This is 100% correct. You enter off Harmon and go to the guard building, circled in red: He looks up your reservation info and guides you to the correct tower, and you check in like a normal hotel