You cannot "fix" the balloon in PokéPark Wii: Pikachu's Adventure. It's a single-use key item for a specific story sequence. Once used (or lost), it's gone—but the game is designed so you never need it again.
PokéPark Wii is linear and forgiving. No permanent missables. No inventory management puzzles. If you lost the balloon, you either haven't used it yet (go back to Meadow Zone) or already did (you're past it). Keep exploring.