Once getting to Joe (his pet I believe) I tried finding Old Mac's corpse. Minutes turned into hours of searching and I finally abandoned the mission.
Has anyone completed it, or could you give me some help/hint?
Finding Old Mac
Sadaigh
Zaor_Irieryl
When I did it the first time with a friend, we were stuck on the same thing, we found no corpse. Turns out we had arrived to late and a Charr had killed him, but somehow Joe had killed the Charr. I suggest you try again and run to his location as fast as you can 
Sadaigh
Aha, weird though. The corpse should still be in the area.. So I need to get to Old Mac alive? 
Zaor_Irieryl
What I did was return to Piken Square and redo quest. Weird was that time, Old Mac was alive, but Joe wasn't, yet Old Mac kept talking like Joe was there. I was like "Oh poor guy, he went nuts" LOL
Sadaigh
Haha, must be hard to lose your pet like that :'(
LordCrono
Same thing happened to me ]=. But i soon figured out y dam Char!