I'm having same problems. I've downloaded various unlocked .img file maps for instance GB Discover 1:50k v2.00 and 3, Cairngorm, Snowdon, National forest, Peak district 1:25k and used jetmouse to get the unlock code. none of the maps show on the unit (Gpsmap 64s) but work in basecamp when the unit is plugged in.

I previously had a 62s whith a lot of unlocked working maps on but unfortunately lost it (long story) so decided to upgrade to the 64s. perhaps should have saved some money and got another 62s???