pt-get install xfree86-driver-synaptics

Then you'll need to modify your /etc/X11/XF86Config-4 to something like below:

Section "ServerLayout"

Identifier "Simple Layout"

Screen "Screen 1"
# InputDevice "Mouse1" "CorePointer"
InputDevice "touchpad" "CorePointer"
InputDevice "Keyboard1" "CoreKeyboard"

Section "InputDevice"
Driver "synaptics"
Identifier "touchpad"
Option "Device" "/dev/psaux"
Option "Protocol" "auto-dev"
Option "LeftEdge" "1900"
Option "RightEdge" "5400"
Option "TopEdge" "1900"
Option "BottomEdge" "4000"
Option "FingerLow" "25"
Option "FingerHigh" "30"
Option "MaxTapTime" "180"
Option "MaxTapMove" "220"
Option "VertScrollDelta" "100"
Option "MinSpeed" "0.02"
Option "MaxSpeed" "0.18"
Option "AccelFactor" "0.0010"
Option "SHMConfig" "on"
# Option "Repeater" "/dev/ps2mouse"
EndSection