From MEPIS Documentation Wiki
Sudo is a program for some Unix and Unix-like computer operating systems that allows users to run programs with the security privileges of another user (normally the superuser, or root).
MEPIS comes with "sudo" installed but not enabled. You need to set up the configuration file /etc/sudoers in order to use programs like TrueCrypt that demand sudo. The easiest and most secure way to do this is to add an entry to allow your user to run just the one command you need, although it is possible (but not recommended in MEPIS) to set it up system-wide.
There is a program installed called visudo for modifying the configuration file which checks for syntax errors and should be used whenever editing that file.
Open a terminal and become root, then type
Edit the file to read like so (replacing "myusername" with your username and "/path/to/truecrypt" with its actual location):
# sudoers file. # # This file MUST be edited with the "visudo" command as root. # # See the man page for details on how to write a sudoers file. # # Host alias specification # User alias specification # Cmnd alias specification # User privilege specification root ALL=(ALL) ALL myusername localhost = /path/to/truecrypt
In visudo, <ctrl>+o saves and <ctrl>+x exits. When you exit, syntax is checked. If you've made any errors they are described, then you are asked "What now?" Press [b]e[/b] to continue editing or [b]?[/b] for other options.
When you are done, then sudo should work for that purpose.
NOTE: You can set sudo up to allow your user to run any command on your machine by using the line
myusername localhost = ALL
Again, this is not recommended in MEPIS because of the loss of security.