ZoHo plug in for MS Outlook

ZoHo plug in for MS Outlook

I have downloaded the ZoHo Plug-In for MS Outlook and am trying to enable it for our users.
 
1.  Click Setup > Subscription Manager > Activate Users.
 
2.  In the Activate Users page, click Edit and enable Zoho CRM Plug-in for Microsoft Outlook checkbox for the required users.
 
The Zoho CRM Plug-In for Microsoft Outlook is not showing up in the Activate User page as an option.
 
Please help.