Open Yast -> Software -> Install and Remove Software
Select filter 'Search'
search for 'kernel-source', check the entry, click 'Accept'
If you ran an Online Update previously, you'll need to run it again after the installation of the kernel sources, to get the same version (kernel and source).
If you have 9.1 Personal edition, the sources are not on the CD and you'll need to download them from any SuSE FTP server.