Copsey's gets my vote. After having issues myself, I've become a damn-near expert in this field (pardon) and actually designed (to code/spec) the drain field I believe I was going to install to replace my failing one. Ultimately found out it wasn't a drain field issue (but more human error).
Anyhow, to locate:
1. Look in your basement/lowest level for your sanitary main. It should be the only pipe running up/down that is 4-6" in diameter. Depending on when your house was built, it is most likely cast iron or something similar. In theory, the main should exit your house directly outside that outside wall and is most likely in your backyard (but doesn't have to be).
2. Another way is to look for the sanitary vent on the roof. Same as #1, this is about where the main should exist the house.
3. Ground is soft. Grab a peice of long rebar and poke around in this area. Most tanks are 8-15' off the exterior of the house. Poke around in the ground until you hit something hard.
4. Rent/consider buying a metal finder. Run this around the edge of your house. It may pick up either the main itself or any metal lid on the tank.
It's worth investing the time/effort to locate the lid yourself. If you have them dig it up, your pump out cost will be much more expensive. Also bringing a backhoe into your hard may tear it up a good bit as the ground is really soft right now.
No matter who does, either purchase or build yourself a neck around the pump out opening so you can put on a new lid and bury. This way, at the next pump out, you simply scrap off some rock/dirt and pull the lids for access.
on this subject, any help would greatly appreciated, thanks in advance.
