You have a file called Briefing.html, right? Put it in the mission folder alongside the mission.sqm. It will be somewhere like
C\... \OFP\users\yourname\missions\missionname
Once you've done that, go back to the Mission Editor, open your mission, press and hold the shift key and click Preview.
If you haven't done so already, head over to the Editors Depot and read snYpir's OFPEC Guide: Making Missions and also macguba's guide to mission editing for beginners.
Edit: Knut Erik wins again ...
Make sure your file is not called briefing.html.txt by mistake, its a common problem.
Oh, and
Welcome to the forum!