you need to backsweeten it, using either juice or concentrate. However, once the extra sugars are added, the yeast will consume them, and dry the beer out. So you need to add more sugars, but stop fermentation.
there are a couple of options for a carbed, sweet cider:
1) the easiest, is to keg it
2) pasteurize it
there is also some great info on this site from people WAY more experienced than me. Try searching for "sweet cider" or 'Carbed Sweet cider" in the search tab on top, it will probably help you more than my crappy post!