It doesnt work like that. DV files are generally one of two standard types and the codec you use simply says how you decode that to show on your screen. The different companies codecs may use different methods and have slightly different results in quality but all use the same standard file format (barring the odd exception)
The matrox codec opens files created with the MSDV codec from things like premier. (remember that capturing the file from your camera is simply a copy operation and no codec at all is used for that)
So your problem is one of two things. Either you have installed the wrong codec or its not installed properly or the file is not a DV file.
aha! Well it said something about the Microsoft DV codec or something like that so I did the usual Matrox install (as seen on the FAQ thread).
The files that I take off my digi-camcorder are in the avi format (MJPG) - and using G-Spot (I think it's called) it said that the codecs needed for that file type are installed...
Only way I've figured so far is to convert the files into another format and use it in Alamdv from there but obviously - some quality is lost and I'd rather not go for the scenic trip if there's a route one!