This is our location -- KI (a furniture showroom) -- 115 Broad Street, Boston, MA. Enter under where it says "115 Broad", walk straight through the foyer on the ground floor.
I have always found parking to be easy to find nearby. The meters are active until 8pm. If you park at 6pm, you'll need 10 quarters (for two hours) to park for the rest of the night.
The lobby outside door locks at 6pm, so come around the corner to the gate on Wendell Street and knock for entrance. (The mouse cursor in the image below is indicating the gate.) http://goo.gl/maps/k4Pr for an interactive view.