In case anyone else is running into issues with a downloaded map not appearing in BaseCamp, here's what worked for me, and an explanation of what is going on under the hood.
Problem: After downloading and installing Colorado Topo 2011 map, BaseCamp did not see the newly installed map listed. My computer is Windows 10, 64 bit. BaseCamp version 4.6.2.
Cause: the cotopo11 installer is broken and does not insert registry keys where BaseCamp expects them*, which is in:
After installing the My Trails map from this site, I found a key, "My Trails" appearing under the Families key. The values and subkey define file locations for this map. No such key appeared corresponding to cotopo11.
Disclaimer: mucking with regedit is dangerous and could wreck your computer.
Using the My Trails hierarchy as a pattern, I created a cotopo11 key. It looks like this:
˪ Default (String Value) left as "value not set"
˪ ID (Binary) I didn't know what value to put in so I guessed--more on this in a sec
˪ TYP (String Value) contains the full path to the TYP file
˪ 1 (Key)
Under the 1 key:
˪ BMAP (String Value) full path of the main img file e.g., cotopo11.img
˪ LOC (String Value) full path containing the many img files, the TYP, TDB, etc.
˪ TDB (String Value) full path to the TDB file
In my case, LOC was set to C:\Program Files (x86)\cotopo11; TYP was set to that same path + 12286.TYP, TDB set to path + cotopo11.tdb; BMAP = path + cotopo11.img.
But the maps won't load in BaseCamp until the ID is corrected.
An easy way to fix this is to download and run (as someone suggested), JaVaWa software. It should find your new map registry keys and notify you that the ID doesn't match the one in the map database. Simply click the new map in the list and click the repair (+) button to fix the bad ID.
After doing this, BaseCamp saw my new map just fine.
The harder way would be to find the ID in the TDB file using a hex editor (HxD, etc.), at bytes 5 and 6. In cotopo11.tdb, those bytes are 68 and 22. You would enter them into the binary ID value as 68 22.
I tried the Utah Topo 2011 map. In the uttopo11.tdb, bytes 5 and 6 were 91 and 22, respectively, and the ID in the registry was set to: 91 22. Though I didn't enter the ID manually (and in fact uttopo11 installed and appeared in basecamp with no intervention), I'm sure this would work.
* I later found that the cotopo11 installer is broken because it creates the registry keys under:
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\WOW6432Node\Garmin\MapSource\Families\cotopo11 -- note the duplicate WOW6432Node key. Oops.