Yes, the MEC is the top union leadership at a given company, they are not the national folks (but an MEC member could be elected to national leadership IIRC).

Each company also has LEC's (local executive councils) which represent pilots in a specific domicile or geographic region. The pilots elect LEC members and then the LEC members elect the MEC members (from their own ranks I think).

I'm not sure how the national leaders are elected, I think it some combination of vote by the total national membership and vote by the group of MEC chairmen from the various airlines.