Basecamp is powerful and confusing. I have never used any of the purchased ACA gps data files so I am going to make some assumptions about those.
First, select the imported gpx file in Basecamp which will be in the left hand column of the screen with your map showing on the greater portion of the screen. Right click with your mouse and select show graph. This should bring an elevation vs. distance graph up as a separate window. If you do not get such a graph my guess is that neither the gpx file or the map you are using in Basecamp contains elevation data.
The default map which comes with Basecamp is called Global Map. This map is almost worthless and only consists of major highways, cities and no elevation data. I think Garmin includes it so that you will not be viewing a blank screen when you start Basecamp. You will need to install a map which contains routing and elevation data. The Garmin City Navigator maps which they will sell you contains this information but probably most people will use some variety of OpenStreetMap.
The map which I prefer to use on my Garmin can be downloaded from here:
http://www.gmaptool.eu/en/content/usa-osm-topo-routableThere is a web page at that site with instructions for installing maps as well. You will need two free Garmin programs to work with maps. Mapmanager will let you install downloaded maps into Basecamp. MapInstall will let you install the map on your Garmin device. There are YouTube tutorials on how to do this if you search there.
There are other sources which provide an OpenStreetMap extraction also.
Once you have installed a better map on Basecamp and your Garmin you should see an elevation profile when you right click and choose Show Graph.
An excellent YouTube series on how to use Basecamp to create routes can be found here:
https://www.youtube.com/c/JosephByrne/playlistsThis is oriented to motorcycle touring, but it's all the same.