just like you would use a context menu... hold down shift and click the item and it will show "load" so select that option then click a stack of shruks... (hopefully you poisoned the stars first)
a belt can hold 10 at a time...
the chance to hit depends on your ninjitsu and if it does hit, your not guaranteed the poison will go off...
also, there are blow guns and darts...
the stars are for throwing at a distance and the gun is for close range...
hope that splains it!