She already knows about the island because of her dad and has somehow figured out that he set up desmond to put him there, he obviously would know what frequency to get the island on and she somehow got the info. but with the blocking going on and the island being cloaked she could niether get there or contact the island, but she constantly had a frequency open and had those guys watching for any activity around where the island should be, so thats how she instantly appeared to Charlie. Just my theory.