Since the USB-UIRT can be controlled by LinuxMCE directly, it can also learn codes directly from the Web Admin site.
To do this, create a new device template by going to Advanced > Configuration > Device Templates
Be sure to pick a manufacturer, or create one if one does not exist.
Then be sure to pick the correct category for your IR device.
After this, you can select [Add Device Template], after which, follow the prompts...be sure to select Infra-red as the communication method. You will then be asked about the capabilities of your device, and systematically have the USB UIRT learn the IR pulses by placing the remote next to the front of its IR window and pressing buttons when prompted to learn.