I had a hard time finding this information for some reason.
If you hold a UPS package for pickup in their Chelmsford, MA location, the address to pick it up is at 90 Brick Kiln Road. (There’s also a UPS HR building there.)
Do not go to the truck depot across the street at 87 Brick Kiln Road. That is not where you pick up packages. (Unless you’re a UPS driver, I suppose.)
If you’re anything like me, this may save you 15 minutes of confusion.