There are nine caches on the island, one of each difficulty level. In order to find the D5 cache, you must first find the other eight, and collect a clue from each, a number in brackets, like this: <3>. For more information about the series, see Eye of the Dragon - T5 Series: T5D5.
You are looking for a different sort of container. BYOP. Thanks to millasmommies12 for changing the clue on the log, which I initially got wrong.