There are two Marian Reforms:
1) The Vanilla one to get which all Vanilla conditions appy, we can't get around that one. Used to trigger some new units for non Roman factions - e.g. Seleukid Kataphraktoi.
2) The EB one, see the FAQ. (Found among the stickies, or just click here. To get the new units follow the instructions listed just below the reforms in the Gameplay section (IV).

Oh, I just forgot to add this smiley: