In Safari 5.x, the procedure for managing tabbed browsing has changed significantly. Please refer to the following updated article: How to Manage Tabbed Browsing in Safari 5 for OS X
Now that you have activated tabbed browsing, it is time to create a new tab.
One way to create a new tab is through Safari's File menu. Simply click "File" from your Safari menu on the top of your screen. When the drop-down menu appears, choose New Tab. If you do not wish to use the menu, the following keyboard shortcut will produce the same results: Apple (Command) + T.