Translation:How do I get to the post office?
Could it be both? There's a verb やる, to do, which in it's -te form looks like やって. Combine it with どう and you could get something meaning how (do you do X) or "in what way." My guess though is that even if that was part of the word some time ago, by now どうやって has become a word of its own.
Is there someone who really knows the answer?
Right, "go" doesn't sound natural here if you are asking for directions. Consider:
Question: How can I get to the concert?
Answer: Take Main Street and it's on the corner of 2nd Street and Main Street.
Question: How can I go to the concert?
Answer: You have to buy a ticket, or know someone in the band.