There is a pedestrian bridge that crosses the street. It's really not that far of a walk. The path starts where the Castaway and Thunderbird buildings meet. Then, it's not far. You just walk along the path beside Sapphire Falls and are there. Make sure to pay attention to the signs. After the bridge, one path heads to Citiwalk, one to Sapphire Falls, and one to Aventura.
Google Maps says the walk from the start of the path to the lobby entrance at Aventura is 7 minutes. If you are on the Volcano Bay side of the CB add approximately 5 minutes to get to the start of the path.