When I set up Channels Plus I had installed a new disk dedicated to DVR recording and so (naively) pointed Channels DVR to the root of the disk (D:)
Later I moved my WMC recordings to the same disk (D:\Recorded TV). I then tried the "Local Content" feature on the web UI Settings and provided this location. The UI seemed to accept the location (and displayed it), but did nothing. I checked the Recordings tab and there was no "Imports" section.
After a bit of experimentation, the light bulb "went off" - I thought there might be an issue with locating foreign/import content inside the DVR root folder. So I re-configured the DVR to "D:\channelsDVR", restarted Channels, and retried the import. Voila, this time it worked, the UI indicated that it was indeed scanning the folder and now there was an Imports section with all of my desired WMC content.
Conclusion:
This was user error for sure, but it may be helpful to at least put a note (if not a more robust error message) in the Local Content UI for that occasional user like me who runs into this e.g. "Please make sure that the folder you are importing is NOT located within the Channels DVR folder".