Whenever I try to use it in pearl, it says I can't.
There could be three problems:
you can't use the pokeradar when you are on a bike
it only works when you are on grass
you haven't recharged your pokeradar
Ah, I know the problem! you can't use it everywhere you want, it's only work when you are in tall grass! go to any tall grass (e.i tall grass on route 202), and then use Pokeradar! if it's work, the grass will shake which contain a wild pokemon...