In case you don't get any upgrade when doing an apt-get update ; apt-get upgrade cycle, check your /etc/apt/preferences file.
It should look similar to this
# We take MythTV from our repo always
Package: mythtv mythtv-frontend mythtv-backend mythtv-common mythtv-database mythtv-transcode-utils mythweb libmythtv-perl libmyth python-myth mythtv-themes mythplugins mytharchive mytharchive-data mythbrowser mythgallery mythgame mythmovies mythmusic mythnetvison mythnetvision-data mythnews mythvideo mythweather mythzoneminder myth-doc
Pin: origin deb.linuxmce.org
# If our local package uses the same version as elsewhere, we take the one we have locally.
If your preference file looks like that, and you still do not get upgrades, even though you think you should, try changing the 600 to 500. Do NOT do this unless you have a problem with the 600. If the 500 does not help, change it back to 600.