It's been a long time since I did this but I believe if you /trade any of the gate guards with an I.M. after his name you should be able to trade crystals to him. These are the same crystals used for crafting (i.e. Fire, Earth, etc.). I don't think you need to trade the crystals in full stacks (they stack to 12) but you will most likely need to trade at least one stack to open the next Rank of missions.
Slaxx Pandemonium zTwistedBrotherz
DRG 95 WAR 95