I don't think this will work as described. DataStax does not provide a JDBC driver, rather it's an api for interacting with Cassandra. You'll want to use the Teiid Connection importer so that Designer can reuse the Cassandra source that you have created for the Teiid server.
Were you able to get connect to Cassandra from Teiid Designer?
1) When using the Designer, create project, then choose "File -> Import -> Teiid Designer -> Teiid Connection Importer"
2) create Cassandra data source, then use cassandra translator, it will take you through steps to create a source model that represents the Cassendra your connection points to.