Stuart's right, but that's one way. When performing add-ons, you can also adjust any of the values at that time. So even though the add-on fee is set to $30 on the Game tab, when you perform add-ons you can change the fee value for any particular add-on.
Also, in version 3.2, currently in beta testing, you can create add-on profiles, each having their own settings (fee, rake, chips, etc). So you can set up multiple add-ons before the tournament. When you perform an add-on, you can quickly select one of the profiles.