Taken from
WiiBrew:
Mega CD emulation
1) Original BIOS ROM files are required (one for each region): unzip & rename them to bios_CD_U.bin, bios_CD_E.bin, bios_CD_J.bin in /genplus/bios/ directory
2) When using ISO image files, WAV or OGG files should be provided as AUDIO track files. You can either load a CUE file pointing to a specific ISO file and containing all the needed information about AUDIO tracks files or directly load the ISO file. In the latter case, the emulator will first look for a .CUE file with the same base name, then if no CUE file is found, it will automatically look for .WAV or .OGG files respecting one of the following naming format (“XXX” being the .ISO filename, NN being the track number and “yyy” being either .wav or .ogg extension): “XXXNN.yyy”, “XXX NN.yyy”, “XXX-NN.yyy”, “XXX - NN.yyy” or “XXX_NN.yyy”. Please note that other compressed audio files (such as .mp3, .mpc, .wma, etc) are NOT supported and audio tracks should be using 16-bit stereo 44100Hz format.
3) When using BIN images files, audio tracks are generally stored within the image file. A CUE file is therefore required with all the needed information about audio tracks location. You can either load the CUE file pointing to the specific BIN file or directly load the .BIN file. In the latter case, the emulator will automatically look for a CUE file with the same base name as the BIN file.