Is it in a different place in this version?
I kinda depends on how you installed Slackware. If you downloaded an ISO and installed from that, odds are you didn't install the kernel source package. If you installed from purchased disks, you MAY have installed the kernel source, in which case it should be in /usr/src.
Not to worry though, if your /usr/src is essentially empty, you can either download the kernel source Slackware package from your favorite Slackware mirror (look in the k folder), or you can get the source from kernel.org and extract it to your /usr/src directory.