I think this is because you need a later operating system to allow this. You can not use the full cloud facilities with OS X 10.5 in the MacBook and need to either buy an OS upgrade ( not too expensive but your macBook may not work on effectively the latest one) or use ICAL via the internet icloud.
In iCal Preferences…, choose Accounts, then select the account that is giving you trouble. Uncheck the box labeled Enable this account. Wait a few seconds, then re-check the box. This should solve the problem.
I'm using OS 10.8.5, there is no box to uncheck in iCAL preferences. Only one account, ICAL...I can only delete it and not sure that is the remedy. Please advise how I can share this public calendar to all. Thanks!