If you don't have the doom.wad file, you need to put it in the same folder as the executable. You will have to use the cp (copy) command in xterm to copy the file from your MMC card. You will type something like:

cp \media\mmc1\doom1.wad \<where doom is installed>\

thats: cp <sourcefile> <destinationfile>

I forgot where it is installed, and I am not near the 770 at the moment. Please don't ask me to email you this file, that is something that you need to get for yourself. You can always download the shareware version of the .wad file - search on Google.