BusyCal allows you to synchronize calendars with iCloud and other CalDAV servers. Once you subscribe to calendars hosted on iCloud any changes you make to events in BusyCal or on iCloud will sync in both directions.
To connect to iCloud (and other CalDAV servers), select Connect to iCloud/CalDAV Server from the Calendar menu and enter your account username and password.
Once you've entered your iCloud account settings, the iCloud server and its calendars appear in the source list and begin syncing. You may now edit events on those calendars and the changes will be synchronized with the server.
Once you are connected to iCloud (or another CalDAV server), you can alter the server settings by control-clicking the server and choosing Server Settings. You may change your username, password, server address, and refresh interval. Push will be enabled by default, if the server supports it.
To remove a server, click the Disconnect button. Any calendars you are subscribed to will be removed from BusyCal, but the calendars will remain intact on the server.