The list that you're seeing is populated by the Recon inventory from your managed Macs, not by what you just added to Casper Admin. In other words, the inventory collection preferences gathers mapped printers across all your systems as part of the receipts collection (if enabled).
Is the Mac you mapped the printers on to upload into Casper Admin also managed, and therefore submitting inventory? If so, if you just recently mapped to those printers, its possible the regular inventory cycle hasn't kicked in yet for that Mac. I'd try running a 'sudo jamf recon' in Terminal and let it complete. then go back to your JSS, refresh and see if the printers now show up in the Receipts section for a Smart Group. You're still also going o see any other printers reported by your managed systems, but the new ones should also show up.

Thanks mm2270. I manually type the name of the printer in the smart group criterion earlier and the one computer I managed to force the printer on has now shown up. No patience.