First, download and install the HPD-20 USB drivers that are compatible with your Mac OS. You can find them available here: https://www.roland.com/global/support/by_product/handsonic_hpd-20/updates_drivers/
***(Be sure to view the detailed information in the MAC OS Driver Block troubleshooting link on the download page).
After the drivers have been installed, connect the USB port on the HPD-20 to your Mac using a standard USB cable, and then turn on the HPD-20.
Within your software, the HPD-20 will appear as a selectable Audio device.