Hoot mon! [yeah, i know it means naught in Scottish Gaelic anyway ]. What you've downloaded is a 'PC Install'. You can load it to your PC and use it to make a partial or complete map for your 270, but you'll need to use an SD card for the full NA img file. Start here for some education: [Only registered and activated users can see links. ] and scroll down to 'Have you downloaded a PC map version (.exe/.msi) for Basecamp/Mapsource?' or you can just copy/paste the gmap folder [see 'Have you downloaded a Gmap folder ?'].
If your head's now spinning, go back to the map thread and look for a ready-to-use img file to download.