Details

1) Download the code from svn:

svn checkout http://coreavc-for-linux.googlecode.com/svn/trunk/ coreavc-for-linux 
cd coreavc-for-linux

2) Install dshowserver following these instructions: DshowserverInstall

3) Install the mythtv patch

cd <path to mythsrc>
patch -p0 < <path to coreavc-for-linux>/mythtv/dshowserver-0.22.patch
make
make install
cp <path to coreavc-for-linux>/mythtv/dshowcodecs ~/.mythtv