There is no HM Mudslide, there never has been. If you mean Surf, then you get it after the 5th gym. The "mudslides" you encounter in the game are actually steep slopes that you need a Mach Bike to get over. You can get the bike at the Bike Shop in Mauville.
The "mudslides" confused me also, at first, especially since the first one you see is sinside that dark cave in Dewford. However, it's hard to show a slope with a 2-D Sprite seen by looking down on it, so you have to give the game designers some credit.
In Third Generation Whirlpool is no longer an HM.
In the Cave of Origin.
up your rump
You get the hm strength and give it to one of your pokemon
it isn't in Pokemon platinum its in ruby and Sapphire
in the cave of origin
Use the hm surf
in the cave of orgin
Get the HM Rock Smash
route 111 but u need rock smash to get to it
Actually, you can't get HM Rock Climb on Pokemon Ruby, its only available in Pokemon Diamond, Pearl and Platinum.
talk with steven in the MOSSDEEP