Read this closely. If you do it wrong, SelfControl will not launch.
To adjust the maximum block time and interval on the sliders, just open Terminal.app in Applications --> Utilities, then enter the following two commands (substituting the words in brackets with numbers). Press enter after each line:
defaults write org.eyebeam.SelfControl MaxBlockLength -int [maximum block length in minutes]
defaults write org.eyebeam.SelfControl BlockLengthInterval -int [block length interval in minutes]