There was a “Refresh iCloud” add-in/button availabled in Microsoft Outlook 2010 on my PC when I setup iCloud to sync with Outlook. After I restart iCloud Control Panel and re-load Outlook, the “Refresh iCloud” button disappeares and I cannot choose calender in iCloud. Try uninstalling both iCloud for PC and Outlook, and reinstalling them, but not working.
How to fix the issue
* On the Windows, open Outlook 2010, go to File > Options > Add-Ins. You can find that an iCloud add-in shows under disabled application add-ins.
* Click on “Manage” > “disabled add-ins” and then enable the iCloud add-in. Then the “Refresh iCloud” will re-appear on the top of your calendar.