Sun Ce is not a playable character in Dynasty Warriors 6 on PlayStation 2. He appears only as a non-playable Wu faction leader during story mode cutscenes and battles. His first playable debut in the mainline series came with Dynasty Warriors 7 and has remained playable in every subsequent entry.
If you're playing the PS2 version and want to use Sun Ce, you'll need to move to a later title or explore mods on PC versions of newer games.