Please follow the below steps to add your REST calendar to iCal
Choose Preferences
1. Click on Accounts
2. Click the PLUS button
Account Type: Choose CalDAV
Enter your REST Account Name
Enter your REST Password
Paste the url for your account provided via REST Support Email
Click Create
You will see that iCal is verifying the connection
Click Continue
From the Accounts window you will see your REST Calendar has been added
Close the iCal Preferences window
View your REST Calendar in iCal