On Windows it is in
"C:\Users\YourUserName\AppData\Roaming\Adobe\Lightroom\Email Address Book\AddressBook.lua"
On a Mac the Address book file name is probably the same "AddressBook.lua". So use Spot Light to search for it. Note the exact path then copy it to the new computer at that same path.
Thanks for the reply, but I don see this path or file on either one of my PC, both are running Win 10. I searched using Cortana and maually looked thru all the possible directories, dont find it.
You might have to turn on the option to show hidden and system files to find that path.
Windows hides all those folder and files by default and stops the built in search system from finding them.
Try some other 3rd party search program.
It is simple. Go to the C drive then Users then your Username. From there you should see a folder called AppData, open it. Then you will see 3 other folders, Local, LocalLow and Roaming. Open Roaming and then Adobe then Lightroom.
If you don't see any of those folders, Users, Your User Name, AppData then they are being hidden from your view.