On other people's Macs in their finder windows there are quick "last 30 days" etc, links to help quickly find recently used files. How do I add this to my finder window?
What Mac OS X version do you have? That smart search feature has gone since Mac OS X 10.7 Lion, but you will probably be able to create your own smart search and add it to your Favourites in the Sidebar.
If you still are on Mac OS X 10.4 Tiger or Mac OS X 10.5 Leopard or Mac OS X 10.6 Snow Leopard, go to Finder > Preferences > Sidebar and check the item you want to show.