Ok, I resolved the problem in an a very circuitous way.
When I double-clicked on the acsm file, an old deleted version of the Sony Reader application would pop up with a message that the file couldn't be found it may have been moved, renamed etc. (Don't ask me why, I don't know.)
As I stated before, if I right clicked and selected ADE to open the file with, ADE would open but no book.
So, I reinstalled the Sony E-Reader software just to see what would happen. I then double-clicked the previously downloaded file and it opens in the new Sony application. I then go and locate where the file is stored in my directory, and see that it's now being stored as an epub file.
I open ADE and use the menu option to add an item, then browse to the Sony E-Reader directory, select the file and now it imports to ADE and from there a drag and a drop to Kobo.
I'm not sure why my Mac defaulted to an old deleted software package when trying to open the file. And, I'm not sure why ADE will not open the acsm file.
What I am sure of is, I have a work around for the problem and that sure is nice.
And thanks to all for the suggestions and advice