How do I add media not in iPhoto to a Keynote presentation?
You can

drag them in from a Finder window

copy and paste from somewhere else

or use Insert &gt; Choose...


You can also add the folders by dragging them into the media browser, as described in Macworld's article Add more folders to the Media Browser.

First, in your iWork program, make sure the tab of the Media Browser that you’d like to modify is selected, then switch to the Finder. Find the folder you’d like to add, and start dragging it. Now press Command-Tab—while still dragging the folder—and switch to your iWork application. When the iWork program becomes active, your floating palettes will show up, and you can then drop the dragged folder into the Media Browser. (You can do the same thing using Exposé instead of Command-Tab, if you prefer.)
To remove folders you’ve added, Control-click on the folder in the Media Browser and select Remove Folder from the pop-up menu. You can only remove top-level folders, not folders within folders, in this manner. To get rid of all the folders you’ve added, Control-click on the Folders folder, and select Remove Folders from the pop-up menu.