(RedHat 5.2 i386) wmconfig apparently no longer works with Window Maker 0.51. It worked with Window Maker 0.50.2. Tried to add wmconfig to the root window using WPrefs.app and it will not work.
WindowMaker-0.51 that is available from contrib.redhat.com and others that I am aware of is compiled with RootWindow Menu support disabled to be Gnome compliant. You will need to obtain the 0.51 source rpm and re-enable root menu support, recompile and install the new rpm package you create.