From Route 2, get off at the Park Ave exit and head north. Go over the crest of the hill, past the big water tower on the right, and then there will be a downward grade for several blocks. Watch for the church on the left. Opposite it on the right will be an apartment building, and the last block before the Mass Ave light will have a lot of diagonal parking along the sides.
By bus, there are several lines that go through Arlington center (at Route 60) and then west on Mass Ave. Bus 62 is a good one; 77 might also work. Try to get the driver to tell you when Park Ave is approaching. Walk one block south, up the hill, and the church is at the intersection with Paul Revere Rd, on the right.
There's also a bus 78 that goes along Park Ave and through Belmont. And there's the MBTA web site for more than you ever wanted to know about Boston-area mass transit.