Either drain a radiator and remove the 1/2" bsp plug/vent and pour it in there .
or if the vents on your rads are not 1/2" bsp you can get inhibitors that come in a mastic type tube and can be squirted in using a standard mastic gun or you can buy a pressure pump (like a garden spray pump) and send it into a drain tap as I usually do
Pete
May 2007