You just need to open the archive file through the Unreal Commander (as a directory by doubleclick or by the Enter button), the program will propose you to install the plug-in.
But if archive does not contain file "pluginst.inf" (it happens so rarely), you need to unpack files from the archive into the some folder (usually to subfolder in "Unreal Commander\Plugins\" folder) and specify file of the plug-in (wcx/wlx/wdx) in the Unreal Commander settings window (on tab "Plugins").