Basically, in order to complete The Grifter secret district in Mafia Wars, you will have to master the jobs in the area. Fortunately, there’s not many of them (just a page, 9 jobs total) and they all have 4 levels of mastery.
There are a few loot items released alongside this secret district too, but the drop rates are really bad especially for the first, Bronze mastery level, so you really have to work hard to get the other mastery levels and consume all that useful energy.
But I guess that the reward – the Grifter itself is really worth the trouble so good luck completing this new secret district in the game!