There is a non-CC LR and a CC-LR so maybe you installed the non-CC version which will ask for a serial number.
The non-CC version is available on the Adobe Updates page: http://www.adobe.com/downloads/updates
The CC version is available as an Install button in the CC Desktop app.
Uninstall the LR 5.7 you have, then log out of the CC Desktop app, reboot, then open the CC Desktop app, and click the Install next to Lightroom.
That worked!!!!! Thanks so much:) Now the question is, why did I spend over an hour on the telephone with a technician the other day, and didn't get the problem solved. It was so simple!
I really appreciate your help.