According to the business card included in Best Buy’s Severance Package of The Office Season 2 DVD, the address of the Dunder Mifflin Scranton Branch is:
1725 Slough Ave.
Scranton, PA 18540
And note the tagline: “Quabity and service since 1949”
Slough Avenue is a nod to the original UK version of The Office, starring Ricky Gervais, which takes place in the town of Slough, Berkshire, England. There is no actual Slough Avenue in Scranton.