HELP! New Motor rear main seal Leaks

Depending on how bad it is you could just leave it, otherwise you need to drop the oil pan and maybe some steering linkage to replace the seal. But it CAN be done with the engine in the car, did it myself once.

My guess is they only pre-lubed the seal with motor oil and after sitting 2 years it got dry and burned up when you started it the first time. Or they put it in wrong which isn't uncommon, I find it works better actually to NOT stagger the two seal halves (one end sticking out slightly) especially if the seals have the locating tabs on the sides.