This is the quest you need to do to activate the mission Gate of Desolation:
http://gw.gamewikis.org/wiki/Crossing_the_Desolation
Is Crossing the Desolation listed in your Quest Log? (hot key for that is "L" or go to the Menu button and hit Quest Log.)
If you do, then it should tell you what steps you have left to do and a green arrow on your compass will point you in the right direction.
If you don't show it, then you should be able to get it from one of the two following people:
Lonai in Sunspear Sanctuary (if you followed Melonni and did the Nundu Bay mission)
OR
Zerai the Learner in The Kodash Bazaar (if you followed Koss and did the Jennur's Horde mission)
Since I already cut and pasted it, here's the wiki summary of the steps you need to take to finish this quest. If you have it, but can't figure out where you are in the process, just keep going to where these people are in the following list and you should be able to find where you left off.
1. Talk to Kormir in the Command Post to find a way across the Desolation.
2. Travel with Zhed to speak with Mirza about crossing the Desolation.
3. Meet Mirza on the outskirts of the Desolation.
4. Return to Kormir.
5. Meet with Kormir at the monument to Palawa Joko's defeat.
6. Examine the monument to Palawa Joko's defeat.
7. Have Zhed help track Palawa Joko. Take Zhed to speak with Dirah Traptail.
8. See Laph Longmane for your reward.
Lastly, you do need to have Zhed Shadowhoof in your party while trying to do the quest, and you must have done either Nundu Bay or Jennur's Horde to get the quest.
Good luck.