Been a long time since I have looked at this. Salar is correct on one thing, those properties are carry over from Solpart's implementation. Jan is also right, in that the document was not updated to reflect what actually got implemented. That said, if you look at the code, you will see that if those properties were implemented all that would happen would be the SeperatorHTML (left/right) included would simply get wrapped in a SPAN tag with a class defined. I'm guessing the reason I commented out the menus ability to do this is someone suggested this was not needed as we are already specifying HTML, which could be its own tag.
So to get the ability to specify class tags for the seperators it should be as simple as specifying your classes in your seperators. Instead of
SeparatorLeftHTML="LEFT" CSSLeftSeparator="myleftseparator"
You code this
SeparatorLeftHTML="<span class=myleftseparator>LEFT</span>"
This gives you complete control over how it gets rendered (you aren't stuck with SPAN tags).