Hi, it might help to know what 'not working' means.
If you can get into the main menus, but it freezes to a blank screen when loading a level, try turning off the music from within the options menu. Go to 'Options > Sound/Music Controls' and turn the 'Music Volume' slider all the way down. If it's already down, but you can hear music playing, try moving it up a notch, then back down again. I find that DScent seems to ignore this setting at startup, and plays music anyway, so you may need to turn the music off each time you play. You should keep the FX and Master Volume settings as they are though, since they don't seem to cause any problems, and it might be rather hard to play without them. : )
If DScent doesn't start at all, you might try making sure you have the right versions of the support files, as I believe there are some versions that aren't compatible. They should be placed in the DScent folder along with the other files, and that folder should be placed in the root of your MicroSD. The DScent.nds file can be placed anywhere on the MicroSD, so you can keep it with your other games, if desired.
Descent is a great game, and runs well on the DS, though you'll probably need to re-map the controls to get the most out of it. It can also help to use a thumb stylus, since reaching the buttons under your stylus hand can be quite difficult otherwise. It's also possible to edit a file to change the on-screen buttons, which might also be useful.