It will take a bit more than that. If you do it that way, you won't have any permissions to write to that directory. You also need to
sudo chown username:users /mnt/files
Personally, I like having my extra partitions right in my home directory, in which case you'd use:
sudo mount /dev/??? /home/username/files *
followed by the chown as illustrated but with "/home/username/files" instead.
* ??? represents the partition in question. Depending on the sort of hard disk you are using, that should be either hda5 or sda5. Since you're still using Mepis6, I would guess the former.