What build of LiteStep are you using? Most of the newer builds have the LSThemeInstaller built in? So if the file is in zip format all you have to do is "Open with..." C:\Litestep\Utilities\LSThemeinstaller.exe

But that is only for OTS2 (Open theme Standard 2) themes, you can find the latest build of litestep here

http://www.litestep.net/download.php?mod=79,module

and if the Theme Installer isn't included you can get it here

http://www.litestep.net/download.php?mod=73,module