It depends on the situation. I don't get paid any sort of salary just for being on the MEC. I am only paid for the time I can't fly due to ALPA work. For example, in February we are scheduled to have an MEC meeting for three days. If I am supposed to work on those days then the company will have to take me off of those trips with pay and the Association will reimburse the company for that time. ALPA calls it Flight Pay Loss (FPL). The Association also pays for any expenses I incur as a result of doing ALPA work (meals, lodging, taxis, cell phone, etc...).
Since ASA pilots are in negotiations, your MEC Chairman and the Negotiating Committee are probably on full-time "buyout" which works a little differently. They won't bid for flying, so it's impossible to pay them the normal FPL based on lost flying. Instead, they pay them on the basis of the maximum credit they could have received if they were flying the line. Basically, they aren't really making any extra money by doing union work. They are actually spending a lot more time at work than a typical line pilot and not getting paid much extra for it. Thank your MEC Chair and NC next time you see them.