Unable to enter altitude on FMS

[Midnight Music 219]

According to the manual:

Assign Flight Level to WPT
- Press the (R)LSK corresponding to the WPT you want to modify and enter the altitude you
want to be at that WPT, then press ENTER key.
 

However, when I press the R LSK key corresponding to the waypoint, nothing happens and I can't enter a new altitude. It works fine on my ATR 72-500 FMS, but not on the ATR 42-500. Am I missing something? Does anyone have a suggestion?