If you plug in the usb stick you will see an entry appear on the left hand panel of the Filemanager.
Click on this entry and find the directory containing the file you want to play
open a terminal window in this directory >tools>open terminal window
type omxplayer -o hdmi filename or omxplayer -o local filename for analogue out
Pi Presents - A toolkit to produce multi-media interactive display applications for museums, visitor centres, and more
Download from http://pipresents.wordpress.com