Here we go, this is part of the xorg.conf file, you will have to edit it by hand on slackware. It is only two lines that need to be added, but just check that your protocol is IMPS/2. Then restart X and you will have a working wheel.
Section "InputDevice"
# Identifier and driver
Identifier "Mouse1"
Driver "mouse"
Option "Protocol" "IMPS/2"
Option "Device" "/dev/mouse"
Option "Buttons" "5"
Option "ZAxisMapping" "4 5"
# Mouse-speed setting for PS/2 mouse.
# Option "Resolution" "256"
# When using XQUEUE, comment out the above two lines, and uncomment
# the following line.
# Option "Protocol" "Xqueue"
# Baudrate and SampleRate are only for some Logitech mice. In
# almost every case these lines should be omitted.
# Option "BaudRate" "9600"
# Option "SampleRate" "150"
# Emulate3Buttons is an option for 2-button Microsoft mice
# Emulate3Timeout is the timeout in milliseconds (default is 50ms)
# Option "Emulate3Buttons"
# Option "Emulate3Timeout" "50"
# ChordMiddle is an option for some 3-button Logitech mice
# Option "ChordMiddle"
EndSection