How does my tire pressure effect the tire's maximum load?
I weigh just over 120KG, and the wheels say that at 50 PSI, the max
  load is 90KG... Does that mean that I should then increase the
  pressure, to increase the maximum load, or decrease it?


The max load on a tire is restricted because higher load can cause a pinch-flat at a given pressure - and the max pressure is limited by the tire properties (and also by the rim strength).

So if you run your tires at maximum recommended pressure 50 PSI, you risk pinching the tube. If you inflate the tire more than 50 PSI, risk of pinch-flat goes down, but the manufacturer does not guarantee that the tire will not explode.

Ideally, you should have selected tires which are specified to bear your weight. In practice, this may be less of an issue because of at least the following:


the vendor might have indicated the "load" as load on just one wheel. Then, as said in the comment, your most loaded wheel is carrying just about 90kg.
1.95" MTB tires are designed for MTB, so there is probably less chance of pinch-flat while riding onroad.


So answering your question: the pressure should be increased, but the tire specification limits the maximum safe increase.