RiceGate. Simply put, this is the easiest way to do it. I had all the same errors as you, but after a few things messed around with here and there, it worked out. Follow these steps.
1. Go to
pastebin
2. Login to your pastebin account.
3. Create a new paste. Make sure its public. Leave all the other settings as they are. Just put something random in the paste for now. Save it, and when you see options like Edit, Copy, Paste, Raw, etc., hit Raw. It's gonna open another page, with nothing except what's in your paste.
4.Now, go to your RMXP project, and run a playtest from the RMXP editor.
5. Go to the Script Editor in RMXP. FInd the script section called PokemonMysteryGift. Change the Url to the URL of the raw pastebin page.
6. Using the debug options, go and create your pokemon/item.
6A. If it is a pokemon, go to your party. Select the pokemon, and hit debug. Scroll down till you see Make Mystery Gift. Follow the steps there.
6B. If it is an item, go to your bag. Find the item, select it, and select Make Mystery Gift. Fill in the details again.
7. Now that you have created your Mystery Gifts, exit out of the menu. Select Debug in the normal ESC Menu( You know, where you can see Bag, Pokemon, Pokedex, etc.)
8. Scroll down till you see Manage Mystery Gifts. It will say searching for Mystery Gifts, and it might bug. Hopefully not though.
9. When it loads, select the Mystery Gift you made earlier from this menu, and select Toggle On/Off. You should now see an X indicating the Mystery Gift is active. Now, go back to the menu, and save your game. If you did everything correctly, you should see a new file in your Game's folder called MysteryGiftMaster.txt. Open it, and copy everything in there.
10. Go back to PasteBin, and replace everything you wrote in the paste with the new text you copied. It might look like a bunch of random stuff, but I think its encrypted.
11. Save your paste. Go into your RMXP game, and just to test, create a new event. Make the graphic something you can interact with. For the events code, put a script code in there, with the code: $Trainer.mysterygiftaccess=true
12. Now load another Playtest. Find the event you put that code in, and it should enable. Save the game, and close it. Start another playtest. Mystery Gift option should sow up at the menu. WHen you select it, it should show the options you toggled on from the toggle on/off.
Lemme know if anything happens.
-Pokegod7020