You need to get your box to download the programme, either by watching it in iPlayer or by starting to watch it and pressing pause until it is completely downloaded.
You then go to the web interface on your Humax that the custom firmware provides and go into the folder that you want to store the stream in, then click on the Save Streamed Content i.e. iPlayer/Youtube button and you'll be prompted for a filename to use. If you give it a few seconds it will also show you details about the stream, the key one being the duration. The next version of the web interface will improve things here so that it's easier to see if the stream is complete.