This is a typical error message when the activation failes:
The error often comes because the sandboxed code service is not running on the SharePoint server. The sandboxed code service has to be started before you can use Calendar Browser for SharePointor any other sandboxed solution.
The SharePoint sandboxed code service is found under Central Administration >Manage services on server.
Please refer to this demo for detailed instructions on the SharePoint sandboxed code service.