A DCR only works with the DECS server application. If you want to connect to RDBMS from a Notes client, NotesSQL, the ODBC driver, will do that.
Unfortunately, IBM's documentation is a jumble due to their attempt to force WebSphere and Composite Applications on the Lotus users a few years back. They started doing that, and we saw Domino Designer get migrated to Eclipse, which caused more confusion. Along with that, many opportunities. And this year, they removed the "Lotus" name from the product, but that change is also incomplete, and contributes to more confusion. The newer development model is focused on OSGi plugins, bundles and XPages. The Domino Designer tooling/documentation has bits of the old and new models in it. But for RDBMS data, you will want to learn about JDBC connectors in Domino.
Start from these slideshows for the more recent approach
http://www-10.lotus.com/ldd/ddwiki.nsf/dx/Video__Using_the_Relational_Controls_in_the_Extension_Library
http://www.idonotes.com/IdoNotes/IdoConnect2013.nsf/dx/show104-buried-treasure-finding-the-hidden-gold-in-lotus-notes-data.htm
And yet another new model is that OpenNTF is hosting a lot of the Skunkworks development of new Domino Designer features, and their releases are not 100% in sync with IBM official releases.
http://www.openntf.org/Internal/home.nsf/blogsAll.xsp