get yourself "Graphstudio" (its included in klite codec pack full).
Then open the mkv file (or whatever file) with Graphstudio. It will show you what codec is used.
If you can see CoreAVC, then the right codec is uses and you just don't see the CoreAvc icon.
You can enable the icon in CoreAvc settings (select "Use Tray Icon"). The icon (blue without CUDA, green with CUDA enabled...) should be visible in the lower right taskmenu.
If Graphstudio shows you a different codec than CoreAvc, then you need to increase the merit of CoreAVC by either selecting in CoreAvc settings "Preferred decoder" or using a codec tool named "Gspot".
In Gspot you can also decrease the currently used Codec so, that AVC is chosen.
Open Gspot and click on tab "System" --> Select "List Coded and other Filters"
In the list find CoreAvc and right click on it and select " Set Filter Merit" and increase the merit for CoreAVC. Here you can certainly search other codecs and decrease the merit, so that CoreAVC is preferred....
Don't forget to update your Nvidia graphicscard to at least 182.05 drivers otherwise CUDA support is greyed out in the CoreAVC settings...
See also following thread: http://ps3mediaserver.org/forum/viewtopic.php?f=2&t=535&hilit=cuda