Which openkm version ? Take a look here for starting http://wiki.openkm.com/index.php/Applic ... figuration
To preview any kind of file must have SWFTools installed, property system.pdf2swf
To preview doc, odt documents etc.. you must have installed open office and configured system.openoffice.path
And finally if you want to preview images must have imagemagick installed and configured system.convert
All parameteres are on OpenKM.cfg, if you're on windows must be windows paths, and after make any change in cfg file remember that must restarting the jboss aplication server