HI and thanks for your question. MDP2 is a free upgrade for MIDI Designer Pro users. If you had bought MDP, it would show up in your Purchased apps.
Instead, you probably unlocked MIDI Designer Lite (which we just released as Version 2.4, a free upgrade for existing users only). Press on the Lite icon in the app and try "Restore Purchases."
Check your receipts from Apple in your email to verify the whole situation, as well.
Thanks!