I have just returned from picking up a Credencial in Lisbon and can give the following information.
Igreja dos Martires. This neoclassical church is on the Rua Garrett near the corner of the Rua Serpa Pinto. If you go by metro, the stop if Baixa/Chiado. You want to take the exit that will take you up to the Chiado, which involves hundreds of feet of escalators up.
When you emerge from the metro exit, you'll be in a very "happening" place. There are a couple of very famous cafes on the square, A Brasileira and Borges. They are old and ornate inside. Head down, and the church is about a block away on your right hand side.
Hours of operation-- I think to be safe, you should plan to go on M-F between 10 and 5. On weekends there are sometimes people there in the office, but not always. The office is on the left hand side of the church as you go towards the altar, about halfway down. Cost is 2E.
I will try to remember to bring my camera the next time I'm in the area and post some pictures, but it is very easy to find.
Laurie