If you're able to load and preview files in the Wave Manager Software, but not able to transfer them to the SPD-SX, it's possible that the USB Mode in the SPD-SX is set incorrectly.
Confirm that the USB Mode is set to "WAVE MGR" with the following steps.
1. Press MENU.
2. Cursor to "SETUP" and then press ENTER.
3. Cursor down to "OPTION," and then press ENTER.
4. Cursor down to "USB MODE."
5. Press the (-) button to select "WAVE MGR."
6. Using the SPD-SX's power button, turn off the power and then on again. The new setting will be applied.