all the info i find on the xs400, including the service manual says that the rear shocks can be adjusted for preload, but i dont see any screws or whatsoever on my bike. how is pre-load adjustment accomplished?

the cam type thing under the spring

merv

At the bottom of the spring there's a collar.

Turn this. Usually it has numbers on it, the higher ones being a stronger setting for bigger loads.

You will likely need a special spanner to do this adjustment.

Can be had from most M/C parts shops, mechanics etc for the horrendously expensive sum of around THREE of your English pounds

Or you may have gotten one included in your bike's tool kit.

called a "C" spanner

It's also much easier to twist the collar to make adjustments if you get the rear tire off the ground unloading the weight off the springs (either by using the center stand, jack under the frame, or a rope tied to rafters etc).