At present, it is impossible for the game to give you a Liberty Ticket from an NPC or anything that will actually allow you to get to Liberty Island. As such, you'll have to plant the wonder card into your save file for it for the island to be accessible. This is necessary for Victini, as it is not catchable anywhere else in Unova.

You will need your .sav file. If you are using a flashcard such as an R4 or an Acekard, then it is likely to be readily available; you will just need to utilize your .sav file. Those of you using DeSmuME will have to use the Export Backup Memory found in the File drop-down, which will export your .dsv as a .sav. Finally, no$gba users will have to use the savegames converter to change the .sav in their BATTERY folder into a regular .sav.

You then need to load up your .sav with the Pokémon Mystery Gift Editor, made by Grovyle91. The setup was included in the download for the patch. If somehow you have this document but NOT the enclosed setup, you can download it here. Once installed, you will also need the .pgf file for the Liberty Ticket event. This also came with the download, but if you are missing it then you can get it from here.

The rest is simple; load up your .sav with the Pokémon Mystery Gift Editor (you don’t even need to take it off the storage device if you’re using a flashcard!) Once the new bit opens, click on Import PGF (change the Current Gift if you happen to already have wonder cards on there) and choose the Liberty Ticket pgf. Once loaded, simply hit the Save button at the top. This will save the Wonder Card to your save file.

Flashcards already have their .sav ready to play, but for the others; DeSmuME users should use the Import Backup Memory under the file tab to bring in the newly edited .sav. Be sure to reset the ROM afterwards for the best results. No$gba users can simply slot their .sav in the BATTERY folder and it should convert back to the no$ format automatically when the ROM is booted.

With the wonder card loaded, simply go into any Pokémon Center and talk to the blue guy on the right. You will get the Liberty Ticket and all required flags will be activated, allowing you to sail on the ship at the far left port of the city to the island. It’s all normal from there. Note that the trainer levels are sufficent for you to catch Victini the moment you first reach Castelia City. 

Don’t let the length of the words fool you; it’s simple!