Go to Iron Island and talk to Riley, he'll give it to you.
Chat with our AI personalities
With hm strenght.
The HM Whirlpool is not obtainable in Pokemon Platinum, for there is no use for it during gameplay.
Strenght is an HM move, teach it to one of your'e pokemon. to use it just select your'e pokemon and select strenght.
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.