Under the Edit menu, open Preference and find the panel on plug-ins and scratch discs. First item lets you designate an additional plug-ins folder. Point it at the place where you have it installed. You may have to restart Photoshop for it to show up.
If this is a very old plug-in, you may need to go to the Extensis web-site and update it. I dimly remember that there was a revision in plug-in formats a number of years back, and some very old ones ceased to work.