The ACA MapSource files come without any maps marked because different members own different map products, and some may not have maps covering the route at all. It is easy to mark the maps you want to send to your GPS receiver (GPSR).
Open the .mps file, zoom out to a comfortable scale like 20 miles, and use the Map tool to mark the maps you want to use along the route. (MapSource draws the screen faster if you press Ctrl + G to hide GPS map detail.) File > Save As the result with a new name and use that to load your GPSR.
Hint: If you download and install the free upgrade to MapSource version 6.5 from the Garmin web site, you can select some routes and tell MapSource to mark all the maps those routes pass through with a single click.
Alternatively, you can load the GPSR in two steps: the maps from one .mps file and the routes and waypoints from a second.
A third alternative: open two instances of MapSource. Copy and paste routes, waypoints, or maps from one to the other.
The .wpt files are for a different program, OziExplorer and a few others that can read its files. MapSource cannot, as you found.
I hope this helps. Please write back if I missed anything.