As an event-only area, the Flower Paradise will only appear if the player has received Oak's Letter. After speaking with Professor Oak on Route 224 and inscribing onto the white rock what the player is most thankful for, the Seabreak Path will appear, and the barren area around the rock will become covered in flowers. A Shaymin (in its Land Forme) will appear from the north, surprising both the player and Professor Oak, and will then venture back. When the player follows it to Flower Paradise, it can be battled, and will be at level 30.
Through exploitation of the Surf glitch and tweaking, players can visit this area without Oak's Letter in Diamond and Pearl, allowing them to encounter Shaymin without having to attend any event. Both glitches have been addressed in Platinum, and Shaymin itself will not appear without the player having Oak's Letter.
Geography
After going through Seabreak Path, the longest route in the Pokémon world, Flower Paradise can be finally found at the end of the long path. Flower Paradise is a huge meadow covered in flowers of various colors. By going up a small hill to the top, Shaymin can be found. It is the only Pokémon to inhabit the area.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
Trivia
Flower Paradise is the northeastern-most location in Sinnoh and is tied with Snowpoint City for the northernmost known location in the Pokémon world.
However, Snowpoint City hosts the northernmost point of access in the Pokémon world, as the entrance of the Snowpoint Temple in Snowpoint City is very slightly north of the northernmost accessible point in the Flower Paradise.
Prior to version 1.1.3 of Pokémon Brilliant Diamond and Shining Pearl, it was possible through a glitch to reach Flower Paradise without Oak's Letter and catch Shaymin. This was patched in version 1.1.3, rendering Flower Paradise currently inaccessible.