After you assign a magnet to the left or right buttock, then click the Properties Tab on the paramater dials. (Make sure your Magnet 1 actor is selected from the props pulldown) On that Properties Tab you'll click the button "Add Element to Deform" and you can then select the hip from your Figure 1 body parts list.

Here's settings I used to give my M3 figure heroic sized glutes. You'll probably have to adjust these setting a bit to work for Hiro or David. There's still a small bit of cleft you can see at the top of the right buttock but may not matter much if you are clothing the figure.

Left Buttock

Mag 1
Scale=120%

Mag Base 1
Scale=18%
xtran=.333
y tran=2.733
z tran=-0.082

Mag Zone 1
Scale=9%
xtran=.333
y tran=3.210
z tran=-0.082

Right Buttock

Mag 1
Scale=120%

Mag Base 1
Scale=18%
xtran=.333
y tran=2.733
z tran=-0.082

Mag Zone 1
Scale=9%
xtran=.333
y tran=3.210
z tran=-0.082

At top is a shot of how it looks, (this disrobed backside is for anatomical purposes and not intended to titillate or offend)