How do I choose the sky?

When I create a new sector, it uses the fsky1 or what ever it's called by default, but when I try to change it, the other sky textures aren't there, they only seem to appear in the sidedef texture list. How do I access them without having to make a weird-ass trigger?

By the way, I'm new here, is that thing about the spelling serious?

I gotta go, I can not develop much.
F_SKY is the texture of the sky. To select a different sky than the classic blue sky, you must create a lump MAINCFG.

You have to take an lumps editor like slumped or XWE to create/edit your MAINCFG. I advise you XWE, after that depends on your tastes.
The infos are here. Good luck to you.

http://wiki.srb2.org/wiki/Level_Header
"SkyNum"is the command to change the sky.

EDIT : I think the spelling should be correct, yes. In any case, no SMS.

Do you want to use a different sky for the whole level altogether or several skies in the same level?

Oh, and the spelling you have right now is just fine. In fact, it's better than that of a lot of other people here.