Well, I'm no Mexican expert--but I'll try to help out with your query.
Both of these cities are in the state of Guerrero along the coast. The capital of Guerrero is Acapulco--also on the coastline--and is one of the top five major beach resorts in Mexico. Zihuantanejo is on the coast about 150 kilometers by car north of Acapulco and Marquelia is about 70 kilometers south of Acapulco, also along the coast. The Ixtapa/Zihuantanejo area is a major resort area in Mexico and Zihuantanejo itself is a fairly large city, while Marquelia is actually quite small--so I guess where you volunteer depends on what you are interested in. Marquelia is thus about 230 kilometers closer to Guatemala than Zihuantanejo is.
The state of Chiapas (at the south end of Mexico) and where you will need to pass through, had some major trouble about two years ago when the state was fighting separatist armies that wanted to make this area a separate country. Nowadays the major problems are in the north of the country, and the southern area of the country is pretty quiet (except out in the Yucatan, which is nowhere close to either of the cities you mentioned.)
You can fly directly into Zihuantanejo by booking a ticket to the Ixtapa/Zihuantanejo airport. In fact, the airport is actually much closer to Zihuantanejo than to Ixtapa (Ixtapa is right on the beach while Zihuantanejo is very slightly inland). Marquelia is much harder to get to, since you will need to fly to Acapulco, and then take a bus south the 70 kilometers to get there. When you take this road, you will pass two large lakes located right near the Pacific Ocean--so I don't know if that is where the turtle refuges are, but I'm guessing that's where they are.
Note: Even from Marquelia, it is still about 500 kilometers at least (and probably a bit farther--maybe 600 kilometers?) to get to Guatemala, and then another 50 kilometers to Guatemala City). You will be in a beautiful part of Mexico, though--that some people believe is one of the nicest part of the whole country. (The town of Puerto Escondido just a few kilometers south of Marquelia is said to be the most beautiful resort city in the country.)
Hope this helps--enjoy
Edit--I don't know anything about the rules for crossing the border each way once you get to Guatemala. You'll have to check with the Mexican consulate or Guatemalan consulate near you for this information. It would seem that flying from Acapulco or Zihuantanejo to Mexico City and then on to Guatemala directly might be the easiest way--rather than taking a very long bus trip.
Here are a weblink that might help in this regard:
http://www.transitionsabroad.com/listings/travel/articles/travel_from_mexico_to_guatemala.shtml
[ Edit: Edited on 30-May-2009, at 14:35 by Calcruzer ]