Open the control panel in ReplicatorG and you should see two checkboxes labeled 'Pressure Valve' and 'Relief Valve'. These two checkboxes control the state of the two solenoids on your extruder. When Pressure Valve is open, the pressure solenoid will be activated.
Seriously - The default on mine was 94 steps/mm (i think). I ended up calculating it to be 98.8 steps/mm.
I did this by putting doing the following (using repetier connected via mini-USB):
heat up the hot-end to 185 C
Retract the filament by 50mm
put a dot on the filament with permanent marker near the filament intake hole. The smaller the better
use the ender's control panel to feed the filament until the dot is level with the filament intake hole.
Retract the filament by 100mm
Using verniers, measure the actual distance the dot has moved away from the filament inlet hole.
Use maths to calculate your new steps/mm
There are better ways of doing this, but i'm retarded and couldn't get enough of a grip on the PTFE tube to pull it out of the hot-end (yes i know about pushing the soft rubber bit simultaneously). Note that this method may be inaccurate as the knurled gear retraces its own previous steps, so don't calibrate it too many times on the same piece of filament if possible.
yes my bed is warped, but this fixed many issues on my first layer, as well as the small gaps between my perimeters and infills. YMMV.