A security policy in place prevents mounting of volumes
Tested on Slackware 12
Probably you dont'have the policy for mounting volumes or the user have no rigths.
Please refer to file
/etc/dbus-1/system.d/hal.conf
the file should contain this lines
<policy group="plugdev">
<allow send_interface="org.freedesktop.Hal.Device.Volume"/>
<allow send_interface="org.freedesktop.Hal.Device.Volume.Crypto"/>
</policy>
<allow send_interface="org.freedesktop.Hal.Device.Volume"/>
<allow send_interface="org.freedesktop.Hal.Device.Volume.Crypto"/>
</policy>
this permit to plugdev group users to mount volumes.
Slackware 12 already has policies and plugdev group, simply add you user(s) to this group