The Addressbook.

Sorry, not yet written. I hope I will get a friend of mine who is speaking english as his native language to translate my first german script soon.
Meanwhile:

Add a new entry using the add button and enter the data (you should at least enter a name).
Save the changes to the database (see edit->change). The changes will only be stored if you say so. This way you can hack around without damaging anything.
If you entered an email address, you can start kmail (if it is installed) using CTRL+M or the menu.

Missing:

Any vCard support.
Drag and Drop support.
A lot of more entry fields.