The "fastest" way to get an e-mail address into your Address Book is to "take" the address from your incoming e-mail.

From the Netscape Mail Window Toolbar..., Choose the message (from the Sender Subject, Date Pane) that has the e-mail address "you want to add" to your e-mail Address Book.

From the Netscape Mail Window Toolbar..., Choose Message, then "click-on" Add to Address Book.

The Address Book Properties Tab will "Pop-up"

Make any "additions" you want to the Fields (Name, E-Mail Address, Description).  
(The use of a "nick name" is optional).

"Click-on" OK

Netscape will include the address in the Address Book..., for future use.