how can I make my old iMac remote work with iTunes on my current Mac

OK, I got the plastic remote from a 2007 MacBook Pro to work with iTunes on my 2010 iMac running Sierra and I think this will work at your end.

First, make sure the batttery in the remote is working--mine was extremely dead:

How to replace the battery in your Apple Remote - Apple Support

Next follow the pairing instructions in this Apple Support article:

macOS Sierra: Pair and unpair an Apple Remote

Once I did that the remote opens iTunes, starts/stops songs, changes tracks and controls the volume. Should be the same on a Mid 2011 model.

Well, we don't know the sub-model of your "current" iMac. That might help if you posted it (from "About this Mac")

The remote function required an infrared receiver in the computer. According to the MacTracker database, that feature disappeared from iMacs with the Late 2011 models. The 21.5 and 27 inch Mid-2011 iMacs were the last that were equipped with the required receiver.

I have two of the A1156 white ones from a couple of Early 2006 Core Duo iMac's.

The Menu button does nothing because there is no Cover Flow, but they still work fine for controlling the Master Volume, Pause/Play, Track fwd&rev on my 2012 Mac Mini running macOS 10.13.3 and iTunes v 12.7.3.

Which is actually pretty surprising, considering that macOS and iTunes are nothing like they were back then.

Is this the older style remote with the white plastic case or the newer style with an silver metal case? I just found my white version, popped in a fresh battery and tried it on my 2010 iMac. It is not happy with the current version of iTunes. Still working on it.

You're welcome!