It depends on how your game is installed. Typically the game file installs go something like C:\Program Files (x86)\Electronic Arts (or Origin Games)\... (could be on a different lettered drive if you have more than one hard drive, Steam installs will be in a different location under Steam).
Then when you get to the place where the game files are actually installed, look for the EP folders (not those for the base game).
...The Sims 3 Island Paradise (or EP10)\GameData\Shared\NonPackaged\Worlds. This last part will always be the same once you are in the correct neighborhood of folders. You know you are in the right place when you find the original Isla Paradiso .world file there as that is the one you will want to save outside of the Program Files and replace it with Ella's.