Head on over to the "Players" page to access the free agent list (any player that is eligible to be added to your roster either as a free agent or via waivers). You will more than likely need to drop a player in order to pick someone up unless you already have an open roster spot available.
ESPN puts together an Undroppable Players list, a grouping of elite players who cannot be dropped to waivers, for two reasons. First let's be honest, it makes you feel great to have guys on your team that can't be dropped because they are just that good. But more importantly it's to insure fair play so that no one can ruin your league by dropping their elite guys to the waiver wire once their team is out of contention, and also so that if you mistakenly try to drop one of these elite players the system won't let you. So to wrap things up, if you can't drop a player and you don't know why there's a good chance they are a member of this special list and apologies, but you can't drop them.