what version of plesk are you running now?
Download the latest autoinstaller for your system and run it. or download the installation files and extract the ports to /usr/ports/swsoft then run make config;make install from that dir.
8.0.1 and it is the latest autoinstaller.
8.1 and 8.1.1 are not available for FreeBSD 6.0 only for 6.1 so probably there is no official plesk support on FreeBSD 6.0 however maybe Plesk 8.1 will works on FreeBSD 6.0 but I don't want to be a beta tester I prefer to take someone experience.
I would just use the 6.1 autoinstaller. shouldnt be many if any differences. Its not hard to cvsup and upgrade 6.0 to 6.1 anyway if it comes down to it.
never mind I aleady update my box to FreeBSD 6.1 and install Plesk 8.1.1 however Plesk fails to install some components and now I have new problem how to install not installed components because using control panel updater I getting some errors.
Here is a new thread http://forum.swsoft.com/showthread.php?s=&threadid=43273
make sure the 8.1.1 ports were already installed to /usr/ports/swsoft which they should have been and make config;make install in the /usr/ports/swsoft dir.
Thanks mate I love you!
I just go to /usr/ports/swsoft/psa-mod-fcgid-configurator/
than type magic 'make install clean' and restart server and woah I have fastcgi component installed.