We have a plug somewhere in the line. We can not find the clean-out for the plumber to access the line. He says find the pipe or he won't even attempt to fix it. Is there a way to fix the problem from inside the house?

I would think you could snake from the vent on the roof, or even safer, remove one of your toilets (closest the clog if you know where it is and snake from there, and then just reseat the toilet (with a new wax ring) when done. Let's see what the experts have to say...

Good advice so far. Check outisde within 18" of the foundation, that's is usually where an clean out exists. Are you main drains PVC? You can always cut the main stack and install a clean out, do not cut into the vent stack unless you can support several hundred lbs of that pipe.