Receipts substitute for registry on macs.
To the OP, it helps to know where you originally installed the game. Not your user files, not an alias, but the game application you double-click to start, where was it on your original installation?
For starters, try this: if you are running Tiger or later, you can use Spotlight to search for any file with "The Sims 2" or "thesims2". Do it for your all folders under your home directory, Shared, and Applications. Empty trash on it all.
Try reinstalling. If that doesn't work, come back and I'll detail the paths where you need to look to uninstall explicitly.
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