The 'intergrated' mission editor is called the 'Skirmish Editor', and is very limited in what you can do.
I recommend AlliED, which is a shareware mission editor that is THE premiere mission utility that exists currently.
Having beta-tested AlliED, and actually still beta testing new versions, AND having tried all other 'discontinued' editors, I have to say that AlliED is very well put together. It's made by Troy Dangerfield, who also made XvtED and X-ED, his two Freeware editors, so you know it's got to be good.
Go to: <a href="http://homepages.picknowl.com.au/troyd/">Troy's Editor Page</a>
"Never regret growing old. 'Tis a privilege denied to many."