Cannot Create Mailing List from Address Book

I am a new user of Thunderbird and trying to migrate Address Books/Mailing Lists from Outlook. I have imported all the email addresses into Thunderbird Address Books via export from Outlook to csv then import. I have followed the process to create a mailing list as described in Thunderbird Help. I have created a mailing list item. I have highlighted the address book entries I want in the mailing list and tried to move them to the mailing list item as described, but the mailing list item is not highlighted and I cannot move the addresses to the mailing list. (It will let me move them to another address book, but that's not what I want.) What am I doing wrong? Thanks.