Sure thing, Calamity...
First off, if you don't already have a description.ext file you'll need to make one.
It's simply a text file with the name changed. Pull out your most convenient text editor (generally notepad or wordpad), and save a text file with that name. Make sure you select 'all files' under the file type box, as otherwise it will add a .txt extension on the end.
The line you're looking to add is:
onLoadIntro="
Your text here"
for the intro/outro, and
onLoadMission="
Your text here"
for the mission.
You can throw that in anywhere and it should do it's thing...