/opt/retropie/configs/pc/emulator.cfg
should look like this
dosbox="%ROM%"
default="dosbox"
then you need to input the individual launch.sh for each game.
/home/pi/RetroPie/roms/pc/Wolfenstein 3D.sh
#!/bin/bash
/opt/retropie/emulators/dosbox/bin/dosbox -c "mount c /home/pi/RetroPie/roms/pc" -c "c:" -c "cd wolf3d" -c "WOLF3D.EXE" -c "exit"
Hope that answers a couple of questions.