I have used the UCANACCESS driver/lib successfully to write data into an Access database on a Windows 7 System. (https://sourceforge.net/projects/ucanaccess/?source=typ_redirect)
I now moved on to a Windows Server 2012 r2 System. I installed the latest (Version 3.0.7) as well as the formally working Version (Version 3.0.4) of UCANACCESS to the path (*/KNIME/jre/lib/ext).
When registering the driver in the KNIME Preference tab the .jar-file is only accepted at a second try. In the Database Connector node the driver did not show up.
When trying the same thing with the JDBC-Driver for MS SQL-Datebases( https://msdn.microsoft.com/de-de/library/mt683464%28v=sql.110%29.aspx ) it works quite well and the driver shows up.
I asume a problem with the driver if the Database Connector does not show the driver name after registering it. Maybe you can try to move the driver file from jre/lib/ext to another location such as a new jdbc folder within the KNIME installation directory. Once done register the driver via the Add Jar button in the database preferences and see if the driver then appears in the driver list of the Database Connector node. If not have a look at the KNIME logfile if a problem with the driver exists. To do so open KNIME and go to View->Open KNIME log. In the logfile view you can search via Ctrl+f. Serach for the string "Load driver from". One of the entries should contain the location of the file and maybe an error message.