... am i meant to just load the editor and leave it open while i fly campaigns missions? if so, what effect does it have on the mod in question?
Just start the Mission Editor when prompted by WOFF (you must enable this feature in WOFF UE Workshop - enable 3rd party mission editor) and (if you want to use ONLY Weather/cloud mods) click on "Weather mod" button located at the top right of ME creen, then apply weather options (wind speed and directions) , close
ME and continue with mission loading in WOFF UE/CFS3. I do not recommend you to keep the ME opened during mission flight because it can have impact on game performance.
However, once you are in Mission Editor, I would recommend you to customize your mission - especially waypoints, because sometimes their placement and selected altitudes are not the best. That is what I always do for my campaign missions. Do not mentioning the feature of "live" and historically correct aircraft placement on airfields with help of "Consolidated airfields mod" by RJW and "Populated airfields mod" by JJJ65. The latter does not work w/o help of Mission editor.
Just my 2 cents.