JOSM's YWMS plugin currently only works in Firefox 2. So if you for example use Debian Lenny and it's got Firefox 3 in the repositories you are screwed. This is how I fixed it.

Note: This might create a mess if you have Firefox 3 installed and use it. I switched to Opera so I don't care. :)

Download the latest http://www.mozilla.com/en-US/firefox/all-older.html
Put it somewhere (for example: ~/projects/openstreetmap/firefox
tar xfvz firefox-2.0.0.15.tar.gz

Now start JOSM, go to the YWMS plugin's settings and set the path to
/home/you/kram/projects/openstreetmap/firefox/firefox
Click the create button for the profile and follow the instructions.