From the interface , main menu, advanced submenu, click on the "Infrared Codes" button.
You will see a list of devices that respond to IR codes. You have to select the device you want and click on advanced button (in the right side).
Then, in the right column you will see the list of available commands and the 'learn button'.
Click on "learn" for the command you want, and you will see the message: "Waiting for IR code". At that moment you have to point your remote at the IR receiver and press the corresponding command button on your remote.
You will see the message stating the command was learnt. Sometimes, it fails to learn it and you have to try again.
There is also another way to input IR commands, by copy-paste them in their specific format (if you find them listed anywhere like this). You can do that from the web admin page/ AV Equipment section, click on AV Properties for the device you want to control.