Happy to help!
You can do this, but it's a bit clunky at the moment.
Toolbar buttons for the built in perspectives that don't already have them is one of the things we'd like to add to a future build of OmniFocus. Folks that want to see this happen should email the
support ninjas so we can attach them to that item in the development database.
In the meantime, you'll essentially want to make a new perspective that clones the All Items one; you can then add that perspective to the toolbar.
Go ahead and select each of the following commands from the menu bar:
Perspectives -> All Items
Perspectives -> Save Window as New Perspective
At this point, the perspectives window will appear, with the item we created in the previous step selected and ready to be renamed. Go ahead and change the name to "All Items", then close the Perspectives window.
Next, hold down the Control key on your keyboard and click in the toolbar; a menu will appear. Select "Customize Toolbar" from that menu.
A sheet will slide out, containing the various things you can add to your OmniFocus toolbar. You should see one that's labeled "All Items"; drag that to your toolbar, press the "Done" button, and you're all set.
Hope this helps!
(I'm going to tweak the title of the thread to try and make this easier for other folks to find.)