Speak with Wharfmaster Dizzywig in Ratchet.
Ratchet is the only port in the Barrens. Most likely Velinde found a trading vessel in Ratchet to take her to Blackwater Cove in Azeroth. We've had limited dealings with the goblins that run the port, but the master of the dock should have information about the comings and goings of ship passengers.
Follow the road southeast through Ashenvale and you will find yourself in the Barrens. Watch your step, [player], warriors of the Horde patrol the land. You will be safe at the port, though.