This is what was posted on the DVD Eastereggs site (as well as my own). I haven't verified it for myself yet...
To get to the Gag Reel, put in disc #6 and from the main menu:
* Select "Special Features"
* Select "The Universe of Babylon 5"
* Select "Data Files"
* Highlight "The Battle for Babylon 5"
* Press down (You should see a highlighted "5" logo appear)
* Press select to view the Gag Reel