I am still new to blender and am working therw the program but cant seem to find this function in the program.
I would love it if some one can show me how to do this.
i.e.
Cut a cube in half, mirror, and have my modling update on the other half.
Make a linked duplicate by ALT+D and an enter right after, without moving the mouse, and then mirror it with CTRL+M and select the axis to mirror it on.
Tip:
If you put the curser at the point you want to be the mirror axis, use . KEY (period) to make it mirror about the curser.
Thanks a lot. 
Just one note to any one who is having this same prob. you need to be in object mode in order for this to work.
Dose any one know how to get this to work in edit mode?
You can’t. The trick works because it creates two objects which share the same mesh data - one of which is flipped and placed alongside.
In new blenders it shouldn’t matter though, as if you go into edit mode with an instanced copy object, all the copies also look in edit mode, and you can select the verts there, etc. So it looks like it’s one mesh (except for a seam down the middle if you smooth the mesh - just join with Ctrl+J and remove doubles with W->Remove Doubles when you’re finished modelling)