kevilay,
There are basically 2 ways to do a copper plane. The one you are describing is basically a copper 'pour' - (Place -> Poweplane...) and this fills the entire plane (minus traces/pads) with a copper region attached to a specific net. It can void - meaning it will not cover existing traces and you can place traces after the copper plane is placed. Also you can remove copper islands, and you should be able to select it (make sure the copper area selection filter is on). I have confirmed this for Ultiboard v11.
The other method is for a polygon shaped area (you manually specify the vertices) - this is called the Copper Area (Place -> Copper Area). Similar to the powerplane it can be attached to a net and can also void (by default voiding is enabled - and you can edit the other properties to specify other characteristics) and therefore you can place traces with other net assignments within and through the plane (and they will automatically have a clearance around them). You can also change the Copper Area shape and delete it.
Is there something I have described about you'd like to do but cannot? Which version are you using?
Regards,
Pat Noonan
National Instruments
