You configure the ARA in extconfig.conf (yes, it's a strange name, but is was defined in the early days of the realtime architecture and kind of stuck).
The part of Asterisk that connects to the ARA use a well defined family name to find the proper database driver. The syntax is easy:
<family> => <realtime driver>,<db name>[,<table>]
The options following the realtime driver identified depends on the driver.
Defined well-known family names are:
Voicemail storage with the support of ODBC described in file docs/odbcstorage.tex (11.1).