I had mixed results getting .wav files onto the mr-8, but a trick I would use is this:
Open up a new project file... record like 9 seconds of silence onto track one. press stop. Plug the MR-8 into your computer. Open up that project file. Find the track where you recorded the 9 seconds of silence. Move your .wav file in there, and rename it that EXACT filename... should be something like MR00033 or some bullshit file name. then delete the old one.
When you open up your file back on the mr-8 it should work.
Let me know if you have a problem.