OK here's my new answer:
All i did was after i beat Eterna gym ( Gardenia ) i walked up to the Dialga statue with My !FRIEND!, When I Was There Cyrus walked off, read the plate, walked into the herb medicine shop (The Shop Was Beside The Statue ),Talk To Everyone In The Shop.Get Out, walk Towards Team Galactic Base ( The Building Beside The Herb Shop )(Go To The Left Tree Near The Fence ) and Cynthia walks up to you then gives it to you.
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Cynthia gives it to you.
The HM Whirlpool is not obtainable in Pokemon Platinum, for there is no use for it during gameplay.
How to get the hm cut in pokemon glaze
Dig is not a HM in Pokemon Platinum but a TM, it can be found in the ruin maniacs cave.
In Pokemon Platinum, Flash is a TM.