Mass Saves was a headache for me as well. I got my 10k rebate in the end and they've since been supportive when we had other troubles with our system.

I called them often and they never answered, then I called on a Saturday morning a little after they opened and someone answered on the first ring. They helped resolve my issue and I received the rebate a few weeks later. Try calling at all sorts of hours and days and see if you have more success.