I just installed the Outlook plug-in for Zoho Calendar.  When the initial sync was performed, my Zoho calendar was blank.  Adding a new appointment to Outlook and then performing another sync will then add that appointment to Zoho.  I am using Outlook 2010.  Testing this on a computer with Outlook 2007 works fine and brings in all already existing appointments during the initial sync.

 

I can't find whether Outlook 2010 is supported and ready to go yet because I find conflicting information at various places on Zoho.com.