You can play them but not use all game features... like create counties or bishoprics etc. In order to play them you need to load your save game in a text editor, look at the beginning of the file where there appears a long list of selectable tags, and you need to add the papal states there. (I'm sure there are instructions somewhere in the mods subforum, otherwise look at the other entries and create a similar one with PAPA as tag...)
When you save a game as them, then you need to modify this saved game again to continue playing the papal states later on.