When importing files using the Wave Manager software within El Capitan, an error message may appear or the Wave Manager may lock up after clicking the "Import" button.
Instead of clicking the Import button, simply *drag and drop* your files into the left-side Wave List menu. If the audio files are the correct format, they will import without an error message or lock up of the software.
Note: Wave files must be the correct format before importing them. Click the link below to see more detailed information about importing files into the SPD-SX: SPD-SX: Notes on Importing WAV or AIFF Files