Hi Dan. The first thing to check is that you installed the correct updateSite to Designer. In the ExtLib zip file there are the following 2 updateSite zips, updateSiteOpenNTF.zip
and updateSiteOpenNTF-designer.zip
. The first is installed to the server, whilst the second should be installed to designer, as it includes designer specific functionality such as the one you describe.
I have release 5 of the ExtLib installed to Designer here, and can see the JDBC Connection manager control in the Data Access palette as expected