Add Addresses to Address Book in Outlook Express
First it is a good idea to display the Address Book (also known and referred to as "Contacts" ) in the lefthand pane just under your Folder List. By double clicking on a contact in this view, we can open the "compose message" dialogue screen AND insert the email address at the same time.

To do this click on "View" on the Menu Bar and select "Layout"
Select "Contacts" by placing a tick in the selection box and clicking OK. This will display the "Contact List" below your "Folder List". You can move the position of the list up or down by selecting the divider bar between the lists and dragging to it's new position.

Use the Scroll Bar to move up and down within the list.
Now to ADD addresses. The easiest way to add an address of someone that has sent you an email is to right click on the sender's email in your INBOX. This will pop up a dropdown menu and all you have to do is to select "Add Sender to Address Book".

That's it and it is that easy !!!
The address will appear in your "Contact List" as well as your Address Book on your tool bar.

To manually add an address (ie. when you have not been sent an email) you can accomplish by using one of two methods.

You can select Address Book on your tool bar, select "New" the select "New Contact" or ...
You can select the down arrow on "Contact" at the top of your "Contact List" displayed in the left pane of Outlook Express, then select "New Contact".
Manually enter the details of the contact by clicking in each field or using your TAB key to switch between fields. (use SHIFT + TAB to navigate backwards through the fields.)

Enter the EMail address then click on the "Add" button. Finish by clicking OK.