Hi,

I have downloaded a quick mission pack called: spitfire2 revenge

I have the following list of files:
spitfire2 revenge.eng
spitfire2 revenge.fra
spitfire2 revenge.ger
spitfire2 revenge.list
spitfire2 revenge.mission
spitfire2 revenge.msnbin
spitfire2 revenge.pol
spitfire2 revenge.rus
spitfire2 revenge.spa

Has it to be installed through BOSEditor?

Finally I found it

Put the files into the "IL-2 Sturmovik Battle of Stalingrad\data\Missions\" folder and the mission name should appear in the game's single mission menu.

You can also create a sub-folder for downloaded missions in the "..\Missions\" directory to better organize your single missions.

Thanks juri_js!