Oh, you are making the skin....
Did you make the button a "Smart Button"?
Or
Just click on the button and "Add Attribute" and add Action on Click...then choose Show System Menu...
Here's what I would do if I was just going to add a System Menu to ANY WB...you can change it do make yours though
Click in ANY button on the left tree and choose dulicate.
Then grab the button and move it into the center of the title bar
Now decide where you want it..on the left right?
Then before you move it change Button Alignment to relative to the top left of window
Ok, and again before you move the button click the next attribute down in the list, this is probaly the Visibility Settings....When you click on it the button will move itself....Now move the button to where you want it...
Then I'd make the Viviblitly Setting to always show..
And then change the "Action on Click" to Show system Menu...
Once that is done, if I want a different button I right click the image I want to use in Explorer and copy it...then go back to Skin Studio and rigth click it's image attribute and import from clipboard...
And if the side of the window already does the system menu change it's Action On CLick to do nothing...
Maybe this helps, if not I'm sure one of the WB skinners will pipe in soon...At least I tried...Good Luck