Simply having the HM is not enough to use it outside of battle. You must meet specific criteria:
: Located near the foggy area, three waterfalls along the river hide valuable items like the Meadow Plate and Wave Incense . pokemon platinum waterfall locations
In Generation 4 (Platinum), Waterfall is an excellent move. Simply having the HM is not enough to
Exploring these areas with Waterfall is crucial for completing your Pokédex and maximizing your team's power. pokemon platinum waterfall locations
Once you have Waterfall, a new tier of exploration opens up immediately.
and speak to (a Gym Leader from the Johto region) . She will give you the HM . Primary Waterfall Locations