You specify the database to be uses with the #db directive:
#db dbname
where dbname is the name of the database that you wish to use.
If you use the -db dbname command tail when you run LIBMAKE, the database specified with the #db directive will be ignored. This is sometimes useful if you have two different generations of a database present and would like to use LIBMAKE to compare the layout of fields within the tables on the two databases.