Go to
http://www.addisonsw.com/regdrop.htm and download regdrop.
make a folder and call it Matroska on c / program files / (example : c/program files/matroska) and put there the .ax file. and the regdrop .
Now drag the .ax and drop it to regdrop :D
Done ! :D
If you want tou unregister the ax filter just Drag it and drop it to regdrop and in the meantime press the SHIFT button !
Bye