Here's how to get it:
1. Beat the game: You need to have completed the main story of Pokémon Pearl.
2. Enter the Grand Underground: After defeating the Elite Four, you'll be able to access the Grand Underground.
3. Find the Resort Area: In the Grand Underground, you'll need to locate the Resort Area. It's the area that looks like a tropical resort.
4. Talk to the NPC: There's an NPC in the Resort Area who will give you the Poké Radar for free.
Note: You can also find the Poké Radar in the Grand Underground in Pokémon Diamond. The process is exactly the same.