Hi mvz,
keep in mind that the Martinair flightschool has absolutely nothing to do with the airline. Actually, I only know of only one former flightschool student who actually went to work for MP.
And as far as applying for a job at Martinair Cargo, there is no such thing. Officially new hires a assigned the A320, however in real life you're placed wherever people are needed. So that could be on any type, from 320 to 744.