Serious? I thought it would be there... It's on line 1,885 of the vB 3.8.7
profile.php page, as
($hook = vBulletinHook::fetch_hook('profile_updateprofile') ) ? eval($hook) : false;
Sorry I couldn't have been more help.
(Just to be sure we're on the same page, you are choosing to "Add New Plugin" and choosing the
profile_updateprofile from the drop-down menu, ya?)