Did you also install the dependencies gnu-unifont, os-prober, and help2man as specified on the Slackbuild page?
I'm just asking in order to cover all the bases.

Yes, I've installed all the deps, I don't know where I'm going wrong?

I'm installing the grub2 slackbuild now - wait a moment

You're right - it's not there

I've just checked on my debian partition as well - it doesn't exist there either.
I think you have to use grub-install.
but check out the documentation using "info grub"
or the man pages, but info seems better
I can't help you much further - I wanted only to clear up the "missing" update-grub, because I remember being confused on the debian machine with this question.

hd0,2 is the third partition. "/dev/sda4 FreeBSD" is "hd0,3" to grub