What problem did you face? Without knowing exactly no-one can help. 
In general if you want to port Directshow library from the full .NET to .NET compact, a ot of things needs to be changed. If you're using a C# wrapper, some of the GUIDs for COM components will require modifications. Most likely, the camera GUIDs are different. If you're using C++ you can re-use most of the simple codes, but more advanced functionalities may not be supported.
The Windows Mobile C SDK is sufficient for directshow apps. There is a simple in C++ telling you how to develop simple Directshow apps. C:\Program Files\Windows Mobile 6 SDK\Samples\PocketPC\CPP\win32\CameraCapture
At present I know no way of making the emulator interface with a webcam. You'll need a real phone (with build-in camera) to do this.
Finally, I guess a better place to post this question would be at the MSDN Forum for smart device. http://forums.microsoft.com/MSDN/default.aspx?ForumGroupID=11&SiteID=1. I guess this forum is not for programming questions.