Hey just a heads up, but for me the description.ext file was not under that directory (took me over an hour of frustrating trial and error to figure it out). It might be because I have Windows 7, installed Arma 2 through Steam, or just have mods like ACE/ACRE installed.
The correct path for me was C:\Libraries\Documents\Arma 2 Other Profiles\[my profile name]\missions\[mission name].
Like I said my computer might just be a unique (frustrating) snowflake, but it might help some others out.