Apparently this function DOES, in fact, return a value as many of the MING examples show. It is a SWFDisplayItem. Here is an excerpt of an example that appears elsewhere in the MING docs...
$p = new SWFSprite();
$i = $p->add($s);
(PHP 4 >= 4.0.5)
SWFSprite::add — Adds an object to a sprite
This function is EXPERIMENTAL. The behaviour of this function, its name, and surrounding documentation may change without notice in a future release of PHP. This function should be used at your own risk.
swfsprite::add() adds a swfshape(), a swfbutton(), a swftext(), a swfaction() or a swfsprite() object.
For displayable types ( swfshape(), swfbutton(), swftext(), swfaction() or swfsprite()), this returns a handle to the object in a display list.
No value is returned.