You'll need to login to the master account for the PS3 to change permissions for any of your other accounts. You'll also need to know the password that was used when the master account was created in order to change these settings.

In other words, if a parent (or someone else) set up the master account, you'll need their password to make any changes. They set things up this way on purpose so that parents wouldn't have to worry about kids changing these settings without permission.