Hi popman:
One reason you might not be able to split a plane is if you placed a Copper Polygon (Place >> Shape >> ANY_SHAPE), instead of a Copper Area (Place >> Copper Area, or Place > Powerplane). It is a little confusing, but you can only split copper areas, not copper polygons.
From the way you the described the expected result, I think you might have misunderstood how the splitter works.
a. After selecting Design >> Polygon Splitter, click where you want to begin the splitting. Then move your mouse, and the splitter will follow your mouse, showing you where the split will occur.
b. Clicking adds additional vertices to the path, so you can define an arbitrary path to split.
You can also change the thickness of each segment, by entering a different value on the Draw Settings toolbar, or right-clicking, and selecting Widden or Narrow.
c. Once you have completed defining the path, press Esc, or right-click and click Cancel.
Garret
Tip: You will notice that by default, you can only split at 45 degree angles. Ultiboard will allow one of four restrictions for placing lines, but the options are not visible on the default user interface.
Select Options >> Customize User Interface... Then on the Commands tab, in the Categories list, select Place. In the Commands list, you will see Place 45 degree lines, Place all angle lines, Place diagonal lines, and Place orthogonal lines. You can drag these to any menu or toolbar you like, and then toggle between modes.
A word of caution, this affects placing all lines (including lines on the silkscreen, and copper traces), so make sure you only allow what you are able to manufacture!
