In speaking with Sir Radzig you have been given his approval to enter the monastery, but there is one condition...
You now have only one option to enter the monastery legally; as a novice monk. To do this you must assume the identity of a young man who is expected to do so. You can find him at the Wagoner's Inn in Sasau.
In order to speak with Karl privately you will have to convince Manfred to leave him alone for a while. The easiest way to do this is by offering him a drink and a game of dice.
Talk to Karl to find out about his story and offer to help him by stealing Guardian Manfred's purse and the writ for Karl to enter the monastery. To do this you will have to sneak into his room at night.
Wait until morning to speak with Brother Porter and formally enter the monastery. Once you have spoken to him, go to the gate house to collect your monk robes and put away all of your other possessions.