Installing new DVD Maps to new SD Card on nuvi 255w

Please forgive my (probably) stupid rookie question, but I've hit a wall. I have a nuvi 255w, preloaded with North America maps. Wanted to add Europe, so installed a new 8GB SD card, purchased the CN Europe NT DVD, installed it onto my Mac, unlocked it successfully to my nuvi. Can't for the life of me figure out how to now get the maps from my Mac onto the SD card. Garmin Express updates the maps, but only to my Mac. I know I'm overlooking something obvious, but FAQ, forum, and Google search didn't help me. What am I missing? TIA for any advice!