I have only installed Slack on RAID devices in Compaq Proliant machines, so I don't know if this is going to work on your machine.
After booting from the cd-rom make sure you select the raid.s kernel. In most cases the drive you want to partition is /dev/ida/c0d0, another possibility would be /dev/rd/c0d0.
omg. sorry. must have been an early morning
Mornings are always a tricky part of the day.