ADDED:
Since you are using PBPL, you can also move the result back to a LONG...
Code:
PRECISION  CON 8 SYSTEM         ' 8 bytes = 64-bit
INCLUDE "N-Bit_Math.pbp"

FOUT    var LONG
Base  VAR BYTE[PRECISION]
Fo    VAR BYTE[PRECISION]
Fs    VAR BYTE[PRECISION]
FTW   VAR BYTE[PRECISION]
FTWL  VAR LONG

high portc.6

FOUT = 10000000

ASM
   MOVE?CP  4294967295, _Base    ; 2^32 - 1
   MOVE?LP  _FOUT, _Fo           ; desired frequency out
   MOVE?CP  360000000, _Fs       ; DDS clock
   
   MATH_MUL _Base, _Fo, _FTW     ; 2^32 * Fo
   MATH_DIV _FTW, _Fs, _FTW      ; / Fs
   MOVE?PL  _FTW, _FTWL          ; move result to a LONG var
ENDASM

serout2 portc.6,84,[" FTW=",HEX8 FTWL," = ",DEC FTWL,13,10]
STOP