Chrissy:
It looks like this parameter has been locked. The best place to look is in the Paramter Script window of the library part, and look for:
LOCK "gs_sill_mat"
If you place an exclamation point ("!") at the start of this line, then ArchiCAD will ignore this command, and the parameter should be unlocked. There may be other consequenses of changing the script.
Looking at the US Library, I found this parameter in the Window dialog box window after setting the Wall Type to Brick Veneer Wall, then going to Brick Veneer > Brick Sill Material (I don't know if the Australian Library is the same), so you might also try experimenting with the window's settings to see if you can unlock the parameter some other way.
HTH
David