That article Jim linked to explains it somewhat, but just to be clear... There are three different ways you can obtain new versions of MC.
1. Via the JRiver Download page here:
http://www.jriver.com/download.html2. Via the automatic updater in MC.
These two (above) options will generally give you a recent, but not the absolutely most current version of Media Center. This is done ON PURPOSE. That's because JRiver actually releases new "builds" of MC about once per week. If they always moved these weekly updates to the auto-update system, then many "regular" users would get very annoyed. It would seem like almost every time you open MC it would ask for an update. If you've used many open source applications you encounter the same problem (every SINGLE time I open Filezilla it has an available update). So, instead of annoying everyone, they only move updates to this auto-updater, and the main download link listed in #1 above, every "once in a while".
It also gives us heavy users (the ones who come to the forums a lot) time to "beat on" the new versions a little and make sure there is no huge bug the beta process missed before it goes out to the larger auto-updating community.
3. The last way you can get MC is via the Interact Forum (here, where you're reading this). This is always the best way to get the absolute most-recent version that is available to the public. What you do is come "here" (to the MC forum) and then look at the threads at the top of the list. One of them, always "pinned" to the top of the list of threads will be named something like "Media Center 15.0.150 -- Available here". If you go to that thread, there will be a download link for the installer for the
most recent version of the software, along with descriptions of what has changed over the last few versions (called a "change log").
Those "pinned" threads that always stay "stuck" to the top of the list on a web forum are called "stickied threads" because they "stick" to the top of the list. If you're curious to learn more about how web forums work and some of their similarities and terminologies, Wikipedia has a nice article about them here:
http://en.wikipedia.org/wiki/Internet_forum