The decomp cable is accuated by the movement of the kick starter via a cam on the kick start shaft inside the clutch cover. There should be a lever just behind/ above the kick start shaft with a cable slot in it. When the kick start lever is rotated, you should see this lever also move. If it does not move, the cam follower may be broken on the inside of the clutch cover. I'm not sure how these cam followers get broken, but when I was at the wreckers looking for parts, I found a few clutch covers, some with broken cam followers.
I've got an '81 XL250, it also did not have a decomp cable fitted when I got the bike a year ago, it was still easy enough to start though, but I wouldn't like to be trying to kick over a 500 without a decomp too many times.
Go to
http://www.cmsnl.com/honda-xl250r-84-us_model841/partslist/E++06.html for a diagramatic.
I've since rebuilt my motor and the decomp works now.
Regards
pmc57