The Somerville Armory is at 191 Highland Avenue.
There is parking behind the Armory, and if that lot is full, there are two nearby lots:
112 Central Street, and 75 Belmont Street, both in Somerville.
You can take the Red Line to Davis Square, and walk (about a mile) along Highland Avenue.
And there are two bus lines that go right by the Armory:
The 88 (Clarendon, Davis, Lechmere)
The 90 (Davis, Sullivan, Assembly)