You can find a lot of missions and campaigns (especially by the Enlightened Florist and DFLion) on airwarfare.com.
DFLion's campaigns go into the 1CSofclub folder for single missions, some screenshots to put into the screenshot folder and some skins to put into the skins folder, all very simple.
The Enlightened Florist's campaigns however need to be put into the folders according to the instructions provided AND at least I had to unblock them so win7 lets me use them ingame. Just right click on the file, go to properties and click unblock.
That is of course after unzipping them and reading the instructions included in the unzipped folder.
Good luck, it works.
