Did your device come with that map or is this an additional map you're trying to install?
Did you also do as suggested, ensure you're running the latest version of Express and delete then reassociate your device, see this FAQ http://support.garmin.com/support/searchSupport/simpleCase.htm?caseId={8a5f1480-ddf1-11e2-7dd9-000000000000}&kbName=garmin