Jean-Max,

You have the 1.7 version of the iPronto's internal software, but you also need the latest version (now this is 2.4.3) of the iProntoEdit software. Download it from the Files section of the RemoteCentral site.
If you install it, take a look at the properties of each device. You are now able to set the hard buttons indivually per device.

With next or previous selected screen I mean this:
First you open the device called TV on your iPronto and after that you open the device called DVD. If you then would have programmed a "Back" button, you would be redirected back to the TV device. If you then press the "Next" button, you are brought back to the DVD device. Just like an internet browser.
This makes navigating between devices easier because you can get to a device with just one click.