Open the config file /etc/default/saslauthd and edit to contain

START=yes
MECHANISMS="pam"
OPTIONS="-c -m /var/spool/postfix/var/run/saslauthd"

We are using pam because we already configured pam to support kerberos. Start the daemon

/etc/init.d/saslauthd start

Now you should already be able to test the authentication with

testsaslauthd -u ${user} -p ${passwd}

and should receive something like "0: OK "Success."".