refdb-mode can be configured entirely through the customization interface of Emacs. Either run the menu command → → or type M-xcustomize and browse to the RefDB section in the Applications section.
If you used the autotools-based package to install refdb-mode, the file /usr/local/etc/refdb-mode-config.el contains a few system-wide customizations which adapt refdb-mode to your local system. ./configure checks for file viewers available on your system, and it also sets the directory containing the global RefDB configuration files. This file is the best place for system-wide refdb-mode customizations.