There are a few wiki pages devoted to getting various types of A/V equipment working, including their remotes. The jist of it is: 1) Add your A/V device through the web admin, 2) Set the A/V device Controlled By to the "USB-UIRT Embedded Emitter" interface that should have been auto-created when you plugged in your USB-UIRT, 3) Go the A/V Properties page of your A/V Device. Once there you should be able to pick from one of the existing IR code sets that match your manufacturer, or create a new one. On the page that follows that you have a list of the various commands that are implemented in your device template. Each one should have the buttons "New", "Test" and "Delete". If you click "New" you will get a pop up window that by default has "Learn New Code" selected. Simply point the remote for your device at the USB-UIRT and hit the buttons that you want to teach. If it works, the window will refresh (sometimes this takes a few seconds) and give you a dialog saying it successfully learnt the code.
There are some good tips and tricks for any troubles you have in teaching remote codes over at the remotecentral site i believe.