ACDSee Pro 3 on the Mac
It appears that ACDSee Pro 3 is using OS X settings. If you right click on a .jpg file in Finder and click on Get Info and then go down to the "Open With" section you can see what apps are associated with that file type and add apps. I verified that it worked with one .jpg file but couldn't get it to change that association for all .jpg files. I'll leave that bit of research to you as I have no intention of using ACDSee Pro 3 even though I did buy it:-)
Well, look at that! Thanks for the pointer. At least now I have some place to start from.
Changing to the Mac has been interesting and a bit of a challenge since I don't know where to look for things like this. I actually asked a question about Approved Applications with the Apple Care folks but they said they did not know of anything like that and pointed me to the ACDSee people.