in dreamweaver2, you cannot position your layers in the center (or any other percentage values) of your page.
one solution would be to code some javascript that checks the window's innerHeight and innerWidth, and the layer's height and width, and then set the layers top position to (innerHeight-height)/2 and the left position to (innerWidth-width)/2.
it will be centered then.. however, this requires some javascript skills, since this has to be done in 2 completely different ways on IE and Netscape...
another solution would be to put the layers in a frame, that has a fixed size, i.e. 500x400 px or so. if that frame is centered, and you position the layer at the correct position, it will be centered in all screen resolutions..
(i think i have that layer-centering javascript stuff around somewhere, email me if you want it --> nico@better-image.nl)