Reading your post it seems that you are trying to configure Mailwasher from within Outlook Express itself. Sorry if I've misunderstood you and ignore the following...
Tools, General Options, tick 'Launch email application after processing'
On the same line there is a 'Specify' button. Click that and in the 'Select Mail Client' box that opens Outlook Express should be available to select in the 'Use a preconfigured mail client' drop down list.