So, you've downloaded Safari 5 to give it a go and you wish you could try out some extensions? There's a little manual hacking to do, but it's as simple as clicking through some menus.
1. click the gear icon on the right side of your Safari menu bar.
2. click preferences.
3. click advanced.
4. check the box marked Show Develop menu in menu bar
5. click the develop menu and then click enable extensions
If you're on Windows, you may have noticed there's no develop menu. Hold down your alt key to display Safari's full menu bar, then click on develop > enable extensions.