Keep reading thread?
You're using a Gateway, correct? Make sure to read required tools at top. You want the "Export eShop to .3DS/.3DZ" section.
Essentially you backup the savedata from the eShop game and import it into a copy and transfer IDs to the copy. Wallah, same game, now on emuNAND (Gateway).
Mount it=I have no idea. Don't own a Gateway. I'm assuming it's telling the GW to "MOUNT" the game into emuNAND the same way you would MOUNT a disk drive, only you're doing it with a file/game and not a drive/hardware.
If anything, use search. Not just Google. GBATemp search "mount (.3DS or .3DZ) gateway" and something should pop up.
It SHOULD be in a sticky post if anything. That's Gateway basics, right? Mounting games?