There are three possible reasons why these check boxes are disabled. The "Last sync peformed:" indicator in the Sync preference pane will tell you why. Here are the three possible reasons:
a) A sync is currently being performed in which case you'll need to wait for it to finish.
b) A sync has never been performed. At least one sync must occur before you can adjust these settings. The first sync will ask you whether you want to sync to contacts, calendars, or notes so you can actually adjust these settings at sync time.
c) Syncing is disabled in iSync's preference window. Launch iSync, open its preference window, and see if "Enable syncing on this computer" is checked off.