Start yast2 and find the package called kernel-sources or something similar to that. It'll be quite large, use the search facility if you can't find it (search for kernel to show all packages with that word in its name). Make sure it's installed. The kernel sources are normally installed to
somewhere like that anyway. Step through the directories until you find the base directory where the source starts. if you can't find it, use kpackage to look at the file list of the kernel sources rpm.
Finally, use an editor (vi or kate will be installed, vi is hard, kate is easy
to edit the Makefile and alter the path (it should be clearly labelled where to edit) to point to that directory.