Outlook plug-in: no button and function available after installation

Outlook plug-in: no button and function available after installation

Hi,
after installation of the MS-outlook plug-in for sync between ZOHO calendar and outlook I can´t find the buttons that should show up on the left side on outlook.
I followed all installation routines (outlook shut) and there was no message about failure or incomplete installation.
Installation process created a folder (C:\Zoho\Zoho Calendar).
Outlook COM-Add-Ins shows ZohoCalendaroladdin as an available add-in, but even after marking it to add it to the starting procedure, nothing happens.
The plug-in will not be loaded to outlook and there is no button available.
I have uninstalled and re-installed several times.
What can I do ?
outlook 2003,
Windows XP.

Please help !
Nico