mapping the flex button can be done in the system settings under the aux switch mappings, and you would want to map the 'performance switch'.
howerever, if you are using an expression pedal, the pitch shifting is automatically assigned to expression pedal as stated before. I would check to make sure that the H9 is set to look for expression pedal and not aux switches.
if you are using H9 Control, these settings can easily be found under the General Settings on the Pedal page.
