Click to make an Icon
Change Icon placement and size and such however you please
Go to triggers and choose an Aura
Set Aura name to Fortitude (the buff you get)
Make sure it's set to Player and Buff
Should be good to go I think, just a quick one from the top of my head if I recall correctly. Icon will appear when it's active and so you'll know when it's up.
On a sidenote, if your raid healers (I'm assuming you're raiding) are competent, you may find the DPS meta to be a delicious piece of kit :P I'm currently tanking with the DPS meta, which appears to scale with attack power. With tank vengeance, that gives a lot of juicy tank deeps. The lowest I've seen that gem give me during a boss fight is about 6 million damage - in fact, I do more damage with the melee dps legendary meta than the melee do.