Building:

place this directory (dialbox) into the following directory of XFree86 4.0.1:
xc/programs/Xserver/hw/xfree86/input/

cd to that dialbox directory, and edit the makefile.
(I haven't made an Imakefile yet, so you have to fix this makefile by hand)
change the "TOP =" entry to the full path to your "xc" directory.

You should get a driver named dialbox_drv.o
Place this with your other XInput driver modules,
and edit your XF86Config with something like:

Section "ServerLayout"
        Identifier "XFree86 Configured"
        Screen      0  "Screen0" 0 0
        InputDevice    "Dialbox0"  # <-- Dialbox entry
        InputDevice    "Mouse0" "CorePointer"
        InputDevice    "Keyboard0" "CoreKeyboard"
EndSection

...

Section "InputDevice"
        Identifier  "Dialbox0"
        Driver      "dialbox"
        Option      "Device" "/dev/ttyS1"
EndSection