Of course, once you have address book entries, you will want to put them into use. Open a new message for composing (See Composing Messages). You'll see header information that you can change (To, Subject, etc.) according to your needs.
Often you will have so many e-mail addresses to know (among other things!), that you simply can't remember them all! That is, of course, where the address book comes in handy. You can quickly add an address, or several addresses, to the To field. Simply click the "Address Book" icon ( ) next to it. A new window will open up with all of your address book entries with a checkbox next to each one of them. Select the address(es) that you wish to send an e-mail to, and click continue. The address(es) will now be in the header criteria that you specified (In this case, To). You can do the same with CC and BCC by clicking their corresponding address book icons. Clicking "Cancel" will completely close the Address Book and abort the operation.
There is also second way to use your address book to automatically add entries to an outgoing e-mail. If you already happen to be in the Address Book Editor, you can click the e-mail address of the entry, and you will be directed to a new message, with that e-mail address in the To field.