can't connect to postgresql

[treaves]

$rcmail_config['db_dsnw'] = 'pgsql://roundcube:roundcube@local/roundcubemail';
$rcmail_config['db_backend'] = 'mdb2';


produces this:

[03-Nov-2006 15:26:13 -0500] DB Error: MDB2 Error: not found in /Library/WebServer/roundcube/program/include/rcube_mdb2.inc on line 103

I know I can connect:

admin > psql -U roundcube roundcubemail
Password for user roundcube:
Welcome to psql 8.1.4, the PostgreSQL interactive terminal.

Type: \copyright for distribution terms
    \h for help with SQL commands
    \? for help with psql commands
    \g or terminate with semicolon to execute query
    \q to quit

roundcubemail=>