Check what state the MODA line is held at for startup as that can stop you from changeing modes.
Also check what addresses you expect to be mapped for the external device. On the device I use (9S12E256) the external memory is mapped in the normal paged area of 0x8000 to 0xBFFF.
The assembly code I use for setting the mode is
; enable read write pin
ldaa #RDWE
staa PEAR
; set mode to normal expanded narrow mode
ldaa #MODC+MODA
staa MODE
; turn on eclk stretching
bset MISC,EXSTR1+EXSTR0
After that if I set PPAGE to '0' then addresses 0x8000 to 0xBFFF are on the external bus.