This question was posted in response to the following article: http://help.adobe.com/en_US/fireworks/cs/extend/WS5b3ccc516d4fbf351e63e3d1183c94856c-7979. html
who do i specify the pixel mode?
That's a fair question. The two modes are "percentage" and "exact".
To determine the answer, I used the Fireworks interface, selecting a rectangle and modifying its Roundness in the Properties Inspector and then using the History panel's "Copy steps to clipboard" command, viewing the results in a text editor.