I'm not sure whether fuses/procedures are the same for 318is's but maybe someone else will confirm.
1. Pull fuse 11 (for fuel pump) and then disconnect the coil wire (or the one that goes to the coil pack on the 318is).
2. Thread compression tester into sparkplug hole
3. Go to driver's seat, and mat the gas pedal to open the throttle.
4. Then crank the engine over until the gauge needle doesn't go any higher. (~3-4 revolutions probably)
Repeat for remaining cylinders.
I think that 2.5i engines should be around 150psi, so I would guess slightly higher would be expected on the higher compression m42, but it is more important to have little variance between cylinders. You usually don't want to have any more than 10% (I think?) difference between each cylinder, less is better.