take the BART to the Powell street station. It’s basically just a 5 minute walk from there. Powell BART will let you off on Market street. Depending on which exit, you’ll either be near 4th st or 5th st. You want to take a right off of Market onto 4th street (past the Old Navy) and walk down 4th street to Mission. take a left and walk one block down mission to 3rd street. MOMA is on 3rd right past Mission. If you look on google maps you’ll get a good sense of the direction – really easy.