As of 1.3.7 the backup functionality moved to setup.pl, the database administration interface.
Login with your ledgersmb database user / admin user. This can't be done using your normal LedgerSMB login account since it doesn't have enough rights to backup db and roles. The default address for setup.pl is http://localhost/ledgersmb/setup.pl.
The setup.pl backup facility consists of two parts:
Backup of the data (parts, customers, accounting records, etc)
Backup of the roles (your login accounts and rights)
The backup can be mailed to an e-mail account, or you can download it with your browser, depending on your output selection in the backup screen.