Somehow the css files that came with the DW CS4 layouts have either disappeared from my computer, or have somehow become disassociated. The result is that I cannot create a new page using one of the provided CS4 layouts

Does anyone have an idea how to get them back?

If you purchased your softwarem then I would try to uninstall it and reinstall it again. OR try to install it to another computer to see if things work for you.