Hmm... back here again with more issues..
Well, I've made a simple platform model in XSI - it looks like this: Problem is, ingame and in Zero Editor, it looks like this: Two of the triangular parts are missing.I did freeze (various things, Freeze M, Freeze & Freeze all Transforms) but as far as I can tell this hasn't made a difference . Every other part of the model is fine - just these triangles seem to 'disappear'.

I think I may have an idea as to what I am doing wrong. To create the flat bridge, I made a cube and scaled it to height 0 (flat). This previously has not given me any issues - but when moving one of the points into another to create a triangle - this is where the poly 'disappears' as you can see. I may not be making these triangles right but I don't really how else to make triangles.

Does anyone have an idea as to what is causing this strange error or how to make flat surfaces and triangles?

Check the following things (I just learned these things myself too)
1) check whether or not your normals are flipped correctly
2) check for identical vertices

I think your problem lies in 1)

1) How exactly would I do this? I'm not the most intelligent person when it comes to XSI. I would however say that I don't understand how the normals would be flipped differently for a triangle than a quadrilateral shape.

(Just found this out myself too)

The normals determine which sides show up ingame. You know that in SWBFII, you can look through the back of models? Well, that's what seems to be happening in you're model too; your faces are flipped the wrong way. Selec the faces that have this problem and try to search for an option called "flip normals". It's probably under some edit menu, I can't tell you for sure since I use Blender myself.
You should also search for an option that says something along the line of "double-sided faces", turn that one off for the entire model.

Easiest way to see if your faces are facing the right way (other that clicking on the Eye menu in any viewport and turning on "show normals" is select your object, press U (polygon racast select) and try and select the polygon in question, if you cannot then the polygon need to be inverted.
To do this rotate the object so you can select the poly(s) in question then on the side menus go to Model/Modify Poly. Mesh/Invert Polygons (from memory so may not be exact) then they should be correct.
If it is the whole object that is wrong then just rotate it 180 degrees in the X or Z plane. If it is a wierd shape and won't rotate nicely try scaling it in the Y plane to a value of -1.

So as you can see there are lots of ways to check and fix this. Have fun.

Oh and as for to how to make flat surface/triangles play around with Slpit Edge, Split Edge with split control or even just add edge. You can also create a grid for your flat surface if you don't need any depth to it like a cube would give.

I pressed U and it turns out that the polies are fine, I can still select the object perfectly well. However I have narrowed it down to a fact that it is only triangles that disappear. I made a flat square from a grid, and then duplicated it, but moved one of the points on top of the other to make a triangle. The square shows up ingame but the triangle however does not.

Could you expand on 'splitting' and edge - and how to use this to make a triangle? Thanks for your help so far, I must have learnt a lot in the past few days .

This is one of many ways to create the platform without thickness:

That's put my platform back in shape. Thanks for everyones help
EDIT: I've also noticed that these are a lot easier to texture things with. Only one surface to deal with. I'm surprised I didn't see this before, it could have saved me a lot of time.