The easiest way to install RPM is to use Slackware's native package manager.
You must be root to install RPM.
installpkg /home/dave/rpm-2.4.12-1.i386.tar.gz
Of course, replace the /home/dave with the correct path for the filename.
(NOTE!) If that fails, simply untar the file with these commands:
cd / ; tar zxvpf /home/dave/rpm-2.4.12-1.i386.tar.gz
Next, you have to create a directory called "rpm" under the /var/lib tree.
mkdir /var/lib/rpm
Now type 'rpm --initdb' to initialize the rpm database.
If everything has gone correctly up to this point, you will have a rpm-capable system! Test it out by grabbing any rpm file and installing it with 'rpm -Uvh filename.rpm'