What you purchased are for sound dampening, not sound proofing, those are two very different things. Dampening, very generally, anything to make the platform stable for all the components. This primarily refers to speakers, like entry-level speakers with cheap cabinets (small, thin wood or plastic to save construction and shipping cost for the manufacturer), but can also include the other components if applicable, like putting your electronics on a sturdy platform. Also, this includes treating the room so the sound doesn't bounce around. Proofing is the one that refers to isolating the sounds inside and outside of the listening area.
You could, as suggested, add Dynamat to the floor (You don't need to use the adhesive back here, just slip and tape them under the carpet rather than make a mess the landlady can complain about later), since the sound transfers through there, but do the math first. Each 1sqm (or so) sheet costs, what, $40? If the cost of sound-proofing that room for a pair of Creative speakers can get you a decent headphone system you really should consider the latter.
As an laternative to Dynamat though, try to see if there are any friendly people in the building with small kids, and try to borrow as many of those rubber puzzle floor mats. You might be able to try them out for free, and they can turn out cheaper than Dynamat (heck you could probably get these cheaper at a garage sale). If you can cover the floor of the room where the speakers are, the rubber will absorb the sound and prevent much of it from reaching the floor, same as Dynamat. The only downside I can see to this is if you can't cover them completely with the rug this **** will be butt ugly with all those young mind-stimulating pink, blue and yellow.