I have seen a few issues with Windows Media Center addins not showing up in the Extra’s gallery, some addins are failing because of compatibility issues related to the .net framework editions but some are failing because they are not registering as an extra in Media Center.
Luckily Dean posted a comment on my Windows 8 post with instructions on how to fix the registrations issues. It works for DVBLink so it should work for other apps that fail to register:
It appears the addin fails during the windows install
if you open an administrator cmd prompt in windows 8 ( right click on the windows start logo, select ‘Command Prompt (Admin)’
then type the following to register the addin in windows 8 Media Center
CD\windows\ehome (press return on Keyboard)
registermceapp c:\progra~2\dvblogic\dvblink\dvblink_addin.xml (press return on keyboard)
you should see it say ‘success’ and the addin will now be installed as normal in media center under extras and works for me.
Hope this helps