For adding a bookmark click on the "+" button on the left of the search field or hit the "⌘ N" Hotkey.
In the window fill the URL field, after that some tags suggested by Delicious.com will appear on the right.
You can insert title, notes, tags and select which account you want to use. You can also mark the bookmark as private so it wont be public.
When you are done you can click Add Bookmark or hit ⌘↩ and the bookmark will be saved on you Delicious.com account. Success will trigger A Growl notificatio, and the color of the system bar icon will change from black to green (or red if an error has occured).