This usually comes from skipping the \make kernel\ step in the instructions. The \make kernel\ is not a sequence telling you to build the kernel, but the actual command to be entered. That is, issue the command for \make\ and build the target called \kernel\.
There are some problems with this logic however. If you are using Slackware 3.0, there is a bug in the \rev\ program with this package. Before the kernel sequence may be patched properly, you must first update the \rev\ program from the file ftp://ftp.cdrom.com/pub/linux/slakware/a8/util.tgz.
It is very important that you do not attempt to replace any file which this package does not replace itself. Do not attempt to force it to replace the ppp.c driver if the \make kernel\ does not wish to do this. There is a date stamp within the files and the files will not be replaced if you currently have a more current version of the driver already in the kernel.
Once the pieces have been installed, please rebuild the kernel at this time. Do this even if you have previously constructed the kernel to support PPP. The driver shipped with the 1.2 and early 1.3 kernels is not compatible with the 2.2 version of pppd.
Once you have rebuilt the kernel then you may resume to build the pppd process, chat, and pppstats.