Really simple. Once you have created a myClub squad you will load in at the myClub dashboard. Once there toggle right to the ‘Contract’ section. You will see the ‘Agents’ tile, select the tile then the game will load you with 2 more tiles ‘Special Agent’ & ‘Top Agent’. Select Special Agent. There you should have 3 options of agent; one being Maradona as a redeemable gift (pink gift box icon in tile) redeem him and the achievement will pop.
***UPDATE***
It would seem the free redeemable agent is no longer available, and you will be required to play matches to earn in game currency or micro-purchase. However do keep an eye out for the above method returning, as there are usually free packs/gifts offered in game for any updates or faults with the game - compensation for server outages etc.