Arma2 SQF Briefings are scripts, which must be executed. Whereas the former HTML versions were read by the engine once it loaded.
You just need to execute your script for it to be seen.
In another script such as Init.sqf for example:
_nul =  execVM "briefing.sqf";
Within the Init field of a unit for example:
nul =  execVM "briefing.sqf"
The difference being the use of a local variable within a script versus a global in the editor.