Wanting to clean debris caught in filter or other partss, Unable to remove center plastic (grey) screw at bottom of assemmbly, it just keeps turning. Any suggestuions?

See if these help: https://dl.dropbox.com/u/8010163/Dishwasher%20Service%20Manuals-Whirlpool-Kenmore.zip
The screw may be stripped, or the piece it screws into is broken. The latter sounds more likely. You may have to replace the whole assembly, and next time put some anti-seize compound on the screw.