Below is troubleshooting information if you're not able to import a .wav file into the #12 memory location on the SPD-1P:
Compatible file type: 16 bit / 44.1kHz .wav [only]. Other file types cannot be used.
Be sure to press and hold the INST/VARIATION button while connecting the SPD-1P to the computer. It will not appear as a storage device without doing this.
Only USB cables that are designed for data transfer can be used. Cables designed for charging only will not work.
Some .wav files may have data embedded data in them [metadata] or they may not be in the correct format [16 bit/44.1kHz] which will prevent importing and/or playback.
With these files, you'll want to prepare them accordingly. You can use iTunes for this: