You can receive the following error at activation time of a plugin for a few reasons:
The plugin generated xxxx characters of unexpected output during activation.
1) There was an error that occurred at activation time on your server (try to capture and output the error message from your PHP script)
2) The PHP files have characters outside the <?php ?> tag
3) Some files got corrupted when you uploaded the plugin.
4) There was some MySQL error on your server.
Try deactivating and reinstalling the plugin. Sometimes characters can get appended to a file when you use the standard FTP transfer. Please transfer using binary mode or just use the WordPress plugin uploader (see the video tutorial from the following link):
Sometimes it could be that the error was shown from a minor warning message. So if you find the plugin to be working as you expect just ignore the message.