Working through Tutorial #1.5, I've come to "When prompted, line select the GPS position and place it into the SET INERTIAL POS field". But after selecting GPS POS into the scratch pad, nothing happens when I click on LSK 5.

 

What have I failed to do?

Did you go back to the previous page before doing LSK5?

Think of the scratchpad as the clipboard for the FMC.  You copy the lat/lon from the GPS then you past it to the INS on a different page. The clipboard holds onto the value for you while you change pages, but you still need to paste it to the intended field.