You're not actually at the airport but in the underwater section which you have to go through to reach the airport. iirc, the route is:
from the initial entrance, take the path going down. Follow it until the area where there is the oghmor colony. You might recognise it from the fact that there is a junction here, with a path going up. Think of it as an upside T. To the right of where the path from above joins, click along that section of the water's edge. I think there's a sort of island in that area as well.
EDIT
Just remembered: see this topic
http://www.aldorlea.org/officialsite/phpBB3/viewtopic.php?f=132&t=6694&p=206572&hilit=orb+of+catharsis#p206572The second post down has a spoiler tag. Open it for a screen shot of the actual location. It is where I thought it was, at the base of that upside T.