NSProgressIndicator isn't specific to Apple - the old OpenStep specification
(from which both Cocoa and GNUstep are derived) had it too.
About the code: I'm not sure whether it's just a typo in the code you pasted,
but the window specification should be like this:
<window title="foo">
<vbox>
<view instanceOf="NSProgressIndicator" height="12"/>
</vbox>
</window>
(without the trailing slash in the <window> opening tag, and the "height"
attribute value quoted)
Anyways, in the case of the "view" tag you should define both the width and
height attributes (or in some form tell the view it's width). That's because
Renaissance can't figure out the correct size of the view to fit it's contents.
Maybe something like the following will help:
<window title="foo">
<vbox>
<view instanceOf="NSProgressIndicator" halign="expand" height="12"/>
</vbox>
</window>