Missing Templar Lairs? D:

I'm having a problem with Templar Lairs on my AC II GOTY edition. I started playing AC II on the normal Xbox 360 version (which was borrowed from a friend) and then switched to the Game of the Year one. I've completed the main storyline, found all chests, bought all weapons & armour, etc. but the Templar Lair icon doesn't appear on my map. I've read countless forums but they were no help. I've found the entrance to the Lair in Santa Maria Gloriosa dei Frari in Venice, but I can't activate the door. And I know this is the right door as it is marked with a large red cross. I've checked the box and found no code to enter on Uplay or anywhere, but knowing me I've probably completely missed it if there is one.
This has been an annoying issue for quite some time now.

Please help?
I would be forever grateful :)