I've felt your pain.
I know the guy's up in Tulsa pretty good myself now. They are really nice but not always helpful. I got Mapcreate 6.3 Topo (Also called Hunting) Is that what you got? I guess you know this pretty good by now, but you create the map border, build the map, and load it on the card. What your saying is that it will not display on the GPS screen? Lowrance says that any GPS unit they make with a MMC/ SD slot will work with 6.3, so it is probably a bug somewhere that you will just have to fix, often without even knowing how you done it. It may be as simple as your computer messing up the file suffix, which should be .LCM and may be displayed as .AT5 in the unit.
Here is a tip, install the product emulator for your H2O off the website, it will usually behave exactly as the actual unit will, without swiching the card back and forth from unit to reader all the time. You can experiment a lot quicker and usually figure out something, if it only how to explain it to the tech guy's. Even if it doesn't help now, you might be glad you have it later, just so you can transfer files and make sure they work.
Good Luck,
AirMapper