Adding multiple thermal relief spokes to a large pad in Mentor-PADS
DISCLAIMER: I'm using PADS VX.2, so our experiences may differ

Here are the steps I followed:

The pad I'll be changing is the one on the bottom left (the square one with thermal spokes.) Seen here is the original pour, similar to what I would imagine you see on your layout.



I then do what you said, which is go into the Properties>Pad Stacks dialog by right-clicking on the pad I want to change. I then apply the settings I want, again in the same manner you describe:



In this case, the only thing I really changed was going from 4 spokes to 7. Note that this is on an inner layer, but I don't think that will make a difference (though you never quite know with Mentor...) Update: It appears that custom thermals only apply to layers defined as "Split/Mixed" in the layer definition dialog, and that defining outer routing/component layers as Split/Mixed can mess up the CAM output in addition to complicating layout and routing due to the way PADS handles split/mixed layers. Thermals for pours on undefined (i.e., non split/mixed or CAM) layers are handled globally and, as far as I can tell, can't be modified without affecting all plane thermals for those pours. If this isn't acceptable, then the thermals may have to be added manually as copper areas.

Now, I hit OK and it seems the critical difference is at the next dialog:



Without selecting "Keep Attributes", I get the same result as you. If I regenerate the pour without selecting "Keep Attributes", then I end up with the default thermal pattern instead of the custom one I defined in the pad stack dialog. However, if I select "Keep Attributes", then I am able to re-pour the layer (by typing SPO and hitting enter, then re-pouring that plane) and I get the pattern I defined: