If you want to center it on one moniter then:
Also I think there is a center window sample in the samples directory.
width=1024 'Your SINGLE MONITER's resolution dimensions here height=768 winw=640 winh=480 winx=(width/2)-(winw/2) winy=(height/2)-(winh/2) window=CreateWindow("Centered Window",winx,winy,winw,winh)
Also I think there is a center window sample in the samples directory.