I too have this problem, but I'm on a MacBook Pro running 10.6.1
I can see the option for my DV Converter (I use the DataVideo DAC-200) in the settings. I can see the video source in the preview within the settings window where you can set the compression, so I know that Quicktime is seeing the DV Converter correctly. I can also use this DV Converter with Final Cut Express without any problems.
But when I return to the main window of Avenue and click on the DV Video Source clip, I get nothing, just black. One time I got a single frame of noise, but this was a fluke as it's never happened since.
I've tried various options in the settings, changing the compression, changing the frame rate, deinterlacing... nothing works.
It is a very strange problem since it seems to work from within the settings panel.
I'm using a MacBook Pro 2.33 GHz Intel Core 2 Duo with 3 GB of RAM
I'm running Mac OS X 10.6
I never used Avenue on OS 10.5 so I don't know if my DV converter would have worked on that OS.
If you need to see the log, I've attached it to this post.