If she is not in the forest, it means she has not teleported yet from Spinel Town. First, make sure it is a clear day, since she only appears then. The lady will be in Spinel Town near the Pokemon Center, like Arom said. Walk around the town for a bit and wait for her to be teleported to the entrance of the forest, by the Tanzan Mountain that leads to that secret cove. Afterwards, well, it is straightforward.