Here are some possible solutions in no particular order:
Check that you have the latest version of the FooEvents plugin and app installed
You must use your website URL (e.g. www.YOURSITE.com) and the access details of one of your WordPress users (e.g. 'admin') to login to the app
Check that your xmlrpc.php file is accessible. Enter www.YOURSITE.com/xmlrpc.php in your web browser. You should see the following message if the xmlrpc.php file is accessible: "XML-RPC server accepts POST requests only."