When I connected this unit to my computer the first time by USB, the installation procedure was canceled before this unit could be used. How do I reinstall it? (The same thing happens when connected to other USB ports on the computer.)

Re-connect the USB cable as such. In some cases, the reinstallation wizard starts up, in other cases it doesn't. If it does, continue installing as such. Follow the wizard to complete installation. If the reinstallation wizard does not start up, install the driver software again.

Pioneer DJ provides the latest firmware and driver software on its web support page. Get them from this page.