Since I dont run X on FreeBSD, I just use it as a server, and with Slackware being similar to FreeBSD, I mistakenly figured it would work.
Doing a quick google on 'IMPS/2' and 'FreeBSD' gave me these results. Which is for FreeBSD 5.3

http://home.nyc.rr.com/computertaijutsu/FreeBSD53.html

"The syntax of xorgconfig (or xf86config) is a bit different than Linux. With an MS optical Intellimouse, using a PS/2 connection, in Linux one chooses IMPS/2 protocol and /dev/psaux. In FreeBSD choose the Auto protocol and use /dev/psm0."