The Truecrypt binary needs to have root privileges. I installed Truecrypt 6.0a on Slackware 12.1 and it gives me the same message. The fix is to create a group with the privilege of running the truecrypt binary and add your unprivileged (non-root) users to the truecrypt group so they can run Truecrypt. Do the following (as root):

# groupadd truecrypt

Now you edit the sudoers file (must be done with vi!)

# visudo

Add the following 2 lines to the bottom of the sudoers file:

# Allow users of truecrypt group to run truecrypt as root.
%truecrypt ALL=(root) NOPASSWD:/usr/bin/truecrypt

Save it and exit visudo Now add your regular users to the truecrypt group

# gpasswd -a regularuser truecrypt

or for many users

# gpasswd -M first_user,second_user,etc truecrypt

Affected users need to log out and back in again.

The Truecrypt file containers default mount on:

/media/truecryptX

where X is the slot number you choose from the graphical interface when you mount the file container.

Mount and unmount with

truecrypt /path/to/encrypted/file /path/to/mount/point
truecrypt -d /path/to/mount/point
truecrypt -d # unmounts all

although mounting in non-default locations seems to cause some serious difficulties (device busy) with unmounting, so I suggest sticking with the default (in /media) unless there's a compelling reason not to.

https://wiki.archlinux.org/index.php/TrueCrypt#Mount_volumes_as_a_normal_user