I used slackbuild to install JMF:
Open
http://slackbuilds.org/ , select your Slackware version and type "jmf".
Download jmf.tar.gz and jmf-2_1_1e-linux-i586.bin pointed at the site.
Extract jmf.tar.gz and put jmf-2_1_1e-linux-i586.bin inside the extracted folder, run the slackbuild script and install the new generated package (# installpkg /tmp/jmf-2.1.1e-i586-1_SBo.tgz or installpkg /tmp/jmf-2.1.1e-i586-1_SBo.txz for Slackware 13.0).