OK, give it a try and see if it springs to life.

I've commented out the line re: set the GIE or INTE bits but left the clearing of the interrupt flag in my handler. Are you sure I don't need that?
Code:
ASM
INT_LIST  macro    ; IntSource,           Label,  Type, ResetFlag?
        INT_Handler    INT_INT, _Flash_Mode_Btn,   PBP,  yes
    endm
    INT_CREATE     ; Creates the interrupt processor
ENDASM
Since you have the ResetFlag "switch" set to Yes, then no you shouldn't have to reset the flag manually. I don't think there's any harm in doing it though, except wasting execution cycles.

/Henrik.