According to Wikipedia app for Windows 8 and Windows RT tablets from Wikimedia blog:
The app integrates with Windows 8′s global search widget, which can be used both to search while in the application and to launch the app — select Wikipedia from the search providers when doing any search and you’ll pop over to us.
On a laptop or desktop device with a keyboard, you can simply start typing to begin a search, just like on Windows’ Start screen.
To open the search “charm” using a mouse (from An Introduction to Charms in Windows 8: What They Are & How to Use Them):
Move your mouse cursor to the top or bottom right corner of the screen, then move the cursor up or down towards the transparent charm icons that appear in the middle of your screen.