I just upgraded my iMac to El Capitan. However, now it won't pair my my MagicTrack trackpad! HELP!

Hello! FORTUNATELY I keep my (antique) USB mouse around for situations like this!

I have an early 2008 iMac, and I just upgraded to El Capitan (the highest I can go). But now it won't find and pair with my Apple MagicTrack trackpad. It worked fine before I started the upgrade...(sigh)

I have the trackpad set to "discoverable" (blinking green light), but my Mac doesn't "find" it. I just put new batteries in, so THAT'S not it. I've tried about 5 times. My Mac keeps "looking" but it's not "finding" anything.

Any suggestions?

THANKS!