To use Yast to install packages, you need to choose "Change source of installation" and enter in the url and path to a mirror. For example, I use LSU's mirror which is hosted at
http://suse.lsu.edu/dist/i386/9.1, so I choose http for the protocol, then enter in suse.lsu.edu for the address and dist/i386/9.1 for the path. For whichever mirror that you choose, make sure to enter in the path for the top level of the 9.1 directory.