Hi!
In answer to your first question, the Fn key opens many different pop-up pages depending on which screen is selected. For the Source page, you need to be on the Processing page and select the Preamp block. In the other processing blocks (Gate, PEQ and Comp), the Fn key shows the Library page.
With regards to your next question, please make sure you do have that channel sourced from the USB bus. If it is and you’re still not hearing the amp, load the file onto a computer and check it there. If you’re still having trouble with this, please start a ticket at support.allen-heath.com for further assistance.
Thanks!
Alex