cabeza, with your hints I managed to find a solution:
First get a cookie from the login and save to a file, in my case I call the file feng
wget -O - --save-cookies feng --post-data "login[username]=myname&login[password]=mypass" "http://server/feng/index.php?c=access&a=login" > /dev/null
Then I download the file using that cookie
wget -O test.html --load-cookies feng "http://server/feng/index.php?c=files&a=download_file&id=18"
Since the username and password is clear text on the command line, I have to wrap it in a script in my multi-user environment, but that is a different story.
Thanks,
Svenn