Do I need something special to test Garmin maps (generated from OSM data) in MapSource, like some license or activation code?
I have installed MapSource and the map inside it (but I can’t use the map, it seems).
Is there a way to test maps without owning a Garmin (or a Garmin map)?
Sorry, I canot understand what you try to ask here. Do you want to view the map, test routing, something else?
Please note that MapSource looks like a rather old program - it seems to have been developed for Windows 95 - expects to be able to write to C:, to HK_Local_Machine of the registry, and other nonsense. I would not try it on Vista or later - if that’s your problem.
You can take a look at Garmin maps on your computer f.e. with QLandkarte GT. But I found out that a map working on the computer mustn’t work on the Garmin device… so no, you cannot test your map without a Garmin.
Maybe I can help if I better explain my needs 
What I want to test is if the map that I am generating with mkgmap is properly assigning the streets to their cities and the cities to their states (ie, if the administrative boundaries are being properly recognized and processed)
How did you install the map in MapSource?
If your map is installed correctly, you can go to View menu > Switch to product, then pick your map from the list. No need for any licence or code.
You can also try Garmin BaseCamp, it is the replacement for MapSource, so is more up to date, and may work better. It uses the same Garmin maps, so any maps installed in MapSource should also be available in BaseCamp.
Also see Javawa GMTK. http://www.javawa.nl/gmtk_en.html
It can tell whether your maps are installed correctly in MapSource/BaseCamp, and warn of possible problems.
Damn… now I am feeling stupid that I didn’t know this… 
Thank you very much for this info!