from the main bus terminal in zihuatanejo, you can take a bus directly to puerto escondido, but be sure to get to the bus station early in the morning. there are only one or two buses a day, and they sell out quickly. we ended up having to take a bus to acapulco, then another to puerto. but the cost is a little more that way. a bus from zihua to puerto will cost about $50. it´s about a 12 hour ride, so i would take an overnight.
The major buscompanies go to puerto from a lot of cities, so where you are just look for a bus that goes there or stops there. There is also an airport where national flights land.
Basicly everything is wiithin walking distance, so that is your best option, or just take a cab, they are cheap.