Hey Dtl62,
Thank you for using the Apple Support Communities! We understand you're trying to use Ecamm Live's virtual webcam with iMovie on your Mac but its not being recognized. We'd like to help.
From what we're seeing on the ecamm support site here Virtual Webcam And Outputs - Ecamm Live User Guide, a subscription might be needed. However, it also states,
Note that some apps don’t load virtual cameras at all. This includes most Apple apps such as FaceTime and iMovie, as well as some third-party apps."
Please reach out to the developer support for your virtual webcam to confirm compatibility.
Best regards.