The first thing you need to find is the force (think of this as the weight) that the plunger (the piece of wood that presses down on the cheese) is putting on the cheese. To do this, do the following:

Measure the distance from the far left pin (it's a bolt, probably--the metal thing on the left that lets the horizontal piece rotate, but not move up or down) to the pin through the plunger (this is the one that is held on with a wing nut--remember the plunger is the wooden piece that pushes down on the cheese). Then measure the distance from the far left pin to the rope on the wooden are. You might get numbers like 10 cm for the first measurement and 30 for the second. Now, divide the second measurement (the bigger one, 30 in my example) by the first measurement (the smaller one, 10 in my example). You'll get a number between 2 and 4 (in my example, I got 3). This is your mechanical advantage. It is the ratio of how much output you get divided by how much input you put in. Take that number, the mechanical advantage and multiply the weight you hang on the rope by it. This is how much "force" you're putting on the cheese. So, in my example, if I hung a 10 kg weight on the string, I'd get the equivalent of 30 kg on the cheese. You know, I'll add a picture here in a minute...

Next, I need to see how much pressure I'm putting on the cheese. This can get a little more complicated because we were just pretending that mass, kg, was weight (which it's not). If you'd like that explained, too, I can do that for you, but why don't you start with this and let us know how it goes.