Yep, "~/" stands for "/Users/Username", but changing it didn't do the trick. Now I'm getting

"Could not load image file: /Users/Bjoern/DOS/DK/GAME.INST
MSCDEX: Failure: Invalid file or unable to open."

Can't say what might be wrong, I know next to nothing about MAC OS. Maybe you'll want to start your own thread, your problem might'll get lost in this one. Or run dosbox the normal way and try first experimenting with commands to see what works on your system so you can then modify your configuration file accordingly. Sorry I couldn't be much help. Good luck.

OK, I managed to solve the "problem": just mount the image.gog! :-)

Sorry for reviving an old topic but as there's no definitive answer I might as well add my solution:

I created a .bat filed in the DK directory (DK.bat) with the following lines:

imgmount d "GAME~1.ins" -t iso -fs iso
keeper.exe

now I simply run dosbox and launch the game via the .bat, works like a charm, no need to use the gog-supplied custom dosbox confs.

If the game's not running smoothly or you have sound issues you can always copy the settings from the custom dosbox conf to your dosbox conf in home/.dosbox

Hope that helps.