Hi,
I write this away from my Softimage computer - so the comments will be broad and general.

Symmetry Mode: Best used when you are transforming (move, scale rotate) your model. Use it to sculpt and shape your existing geometry

If you want to add geometry on the fly and have that update... (e.g. subdivide the edge)
You need to set this up first...
1) delete half the primitive you are starting with
2) create an instance of it (this is a copy with a link to the original)
3) 'mirror it' (in XSI we use SCALE -1 as the mirror command)
(- with the instance selected, choose the axis, probably X, scale that -1, you may want to consider freezing the transform...)
4) start modelling... (if you add edge loops, etc to original, the instance will update, ...)

When done, merge the two halves to form the model, now tweak further in symmetry mode...

L