How do I turn ON/OFF Last 10 PMs??
 

There is no default feature like that. If you have it, it's from a modification. Look at your installed modifications under your User CP to find it.

Then do a search in the modifications forums. If you go to the Add-Ons forum (I'm guessing it's not a template mod), then select Private Message Enhancements or Administrative Options maybe? and look for it there. Then ask your question in the modification thread.

I just removed your thread in the modification forums. You need to post IN the modification thread. Go find the thread and ask your question there. Only modification threads are supposed to be in those forums.

lynne has said she cant direct you to the modification, might i suggest the small box to the top left of the forum aptly named search, you might have some luck there

did you install the modification?

Probably one of these forums then use the Prefix selector at the top to narrow down your search:
vBulletin 3.8 Add-ons
vBulletin 3.7 Add-ons
vBulletin 3.6 Add-ons
vBulletin 3.5 Add-ons