Are you trying to call this over http instead of https? That would cause failure, though I don't think it would cause this failure. Maybe you've gotten prod and sandbox creds mixed...
1. Only custom activities can be written directly with the REST API2. http://developers.marketo.com/rest-api/endpoint-reference/lead-database-endpoint-reference/#/Activities/addCus...
It looks like you have a Spark type subscription which does not have SOAP or REST APIs, or editable user roles. You'll probably need to talk to your account manager about API acces...