Sound from your neighbor below above or on the side of you can drive you batty when living in a condo. Mass Loaded Vinyl has been something that’s been touted around a lot. Considering the difficulty of doing any type of construction in a condo it may be your only choice. Just lay it down on the floor and cover it with a carpet and pad, the thicker the pad the better it will dampen your footsteps (but it won’t help stop the sound coming from below, the MLV will). However if you are trying to stop the sound coming into your walls or ceiling the simplest solution would be to add a 2nd layer of drywall with a damping compound in between . Read a more detailed soundproofing article on this subject.