The default username/password is admin/password, so try this first.
The username and password is stored in an SQL Lite database. There's a couple of easy ways to retrieve them from the database.
Windows: C:\Program Files\MailWasher Enterprise Server\data
1. Get the free http://www.sqliteexpert.com/download.html and navigate to the configure table.
2. Install the Firefox SQL lite browser: https://addons.mozilla.org/en-US/firefox/addon/5817
Navigate to the installed program directory and locate mwes.db (you might need to 'Show all Files' when browsing), then navigate to the configure table >> name, and locate the username and password fields.