It's a little uncomfortable to play the game with the arrow keys. Is there any way to map them to the WASD keys on a Macbook?

Ctrl-F1?

In-game, Command key + K should bring up the keyboard mapper.

Nothing happens when I hit Cmd-K. Should the game be in a certain state for the mapper to be able to start up?

After some research, it doesn't look like it's that easy after all :-/ Cmd+K opened the keymapper in the older version of Boxer. Afaik, GOG's Magic Carpet version uses Boxer (a Mac equivalent of DOSBox) to run the game. Unfortunately the author of Boxer removed the keymapper, probably for good, so no way for remapping the easy way.
What you could try is installing the OSX version of and transferring the game's files to DOSBox' DOSGAMES folder and run the game using the DOSBox environment. Easier said than done, because you'll have to get used to DOSBox ([url=http://www.dosbox.com/DOSBoxManual.html]DOSBox' manual) and you have to identify the location of your game's files to copy them to the new location.

Now run the game within DOSBox and try activating the keymapper with Cmd-F1 or Fn-Cmd-F1. If it doesn't work, what sometimes happens on OSX, locate DOSBox' mapperfile, usually ~/Library/Preferences/mapper-0.74.map and open it with any text editor. There you can remap the keys manually, but you need to know the key numbers of the keys you want use.

All in all, a lot to do for just remapping some keys and it's not 100% guaranteed it'll work, as there are problems with keymapping under OSX (probably the reason why Boxer has this function removed).