The card is probably still too big. The original Ugobe pleos can't read SD cards marked as SDHC (High Capacity or FAT32) or SD cards marked as SDXC (exFAT). Standard SD cards must be 2GB or smaller, but SDHC can be found on smaller capacity cards as low as 128MB, and has been on the market since 2006. Check the specs on the card and make sure it's not an SDHC.
There were "display" models made for retail stores. These were powered by a wire that ran from the back foot down into the display case. The POP OS was designed specifically for these display models. If your pleos run on batteries and don't have the wire, they are probably regular pleos purchased for the exhibit.