Ooops, forgot about that post. Sorry.
You are correct. I have lots of maps in Mapsource, both routable and non-routable, and can confirm that that selection is greyed out when I select non-routable maps like Ibycus' Canada topo but is available for routable maps.
Mapsource has some awareness of "Here" based on what you are doing on the map. So, for instance, if you select a waypoint in the Waypoints tab and right-click, show on map, you have now changed "Here" to the vicinity of that waypoint. If you then select "Find Nearest", the map will stay on that view and your search will be refocused to "nearest to" the waypoint you have selected.
If you select a track file on the Tracks tab and right-click, show on map, you will also change "Here" to the vicinity of the track.
I haven't checked but if you don't have any waypoints or track files, if you scroll the map to the vicinity of where you want to search and then create a waypoint it may cause Mapsource to refocus. Then you can delete the waypoint when you're done the search.
I hope that helps a little.
I'm not sure where you can get routable maps not from Garmin. Most of the free maps do not contain the necessary databases for searching. That's a lot of work to add those. I think Dale is working on a searchable version of the Canadian Ibycus topo map for sometime in the future because there are sources for the necessary data online for free. I'm not sure if the necessary data exists, or is easily acquired, for the American Ibycus maps.
...ken...