Wow, welcome back Campbell. Been a while since you were around the site.
Could you clarify that first question? Sealteam 6 takedown ____?
If a campaign folder does not exist in your Arma2 directory(C:\Program Files\Bohemia Interactive\ArmA 2), create one and place your capaign pbo there.
There are no official single player missions for the expansions but there are campaigns. Crimson Lance created for BAF and Black Gauntlet created fro PMC. There are no doubt, many user made missions out there that you can try out, finished and unfinished. You could have a go through our beta forums and offer some feedback to those mission makers and help them out.
Good to have you back with us pal.