Hi Mark, could you please post a 3DM model file with the object you are trying to split?

It is really hard to tell what is happening to you without actually looking at the model file.

But you do use either Trim or boolean difference to cut up objects.

If you want to cut a solid into multiple solid parts, use boolean difference for that - Trim works by slicing the outside surface of the object into surface fragments. Booleans work by cutting volumes up into smaller volumes.

Also if you want to make a straight planar cut, you can actually just draw a line as the cutting object in one of the side views, rather than making a plane.

Here's a quick demo of splitting an object - here I have a sphere I want to split, and I have drawn a line in the front view as the cutting object. Then I run Construct / Boolean / Difference and pick the line as the cutting object and as you can see it slices the sphere up into 2 smaller solid pieces:




If you haven't already given them a look, I'd recommend checking out the video tutorials here:
http://moi3d.com/1.0/docs/tutorials.htm

They go over a lot of the basic stuff like how to cut objects, etc... - I think that using a boolean to cut an object with a line is covered in the "Modeling a crown of clubs" one if I remember right.

Those videos are not too long and really give a good introduction on how to do these things.

- Michael