Hi,
I think you are getting mixed up with terms and what playerprefs are.
-- to load a level, you use the action "load level Num"
-- to know the level to load, you use the action "Player Prefs Get Int"
you are responsible to decide the playerPref key for the level num, let's assume you choose "Last Level"
so when your use reaches the end of a level and move to next level, you use the action "Player Prefs Set In", and use "Last Level" as the key and the int represent the level num.
Then, you have your logic in place, to retrieve the last level loaded and load it if necessary.
you should study one of the sample provided with ArrayMaker, it's a check point system very much like what you want to achieve in terme of logic.
http://hutonggames.com/playmakerforum/index.php?topic=3546.msg16317#msg16317bye,
Jean