The regular way
First make sure you have The Sims 2 Seasons installed, next get your garden score up to 700 or more by keeping a large well kept garden, I suggest hiring a gardener or your sim will always be gardening. Call for services and select garden club as soon as your score is 700+ they will give you the wishing well. It’s not as simple as it sounds, but I believe you can use buy mode flowers to build up your garden.
The cheating way
This is the easiest and fastest way. First hit the ctrl, shift, and c keys, type in the box that comes up boolproptestingcheatsenabled true shift click your sim and hit spawn and wishing well. It’s as simple as that
What your sim can wish for
a sack of §1000 will fall from the sky, or a sack of §50 will land on your sims head wich may kill them.
3 Sims from the wisher’s age group will materialize near the well with high Daily and Lifetime relationship toward the sim, or 3 sims will appear and pick fights with your sim.
A new love of the right age and preferred gender materializes for an immediate first kiss, or Mrs. Crumplebottom will appear and beat your sim with her purse