There seems to be a small hitch with purchasing the Cow Plant at this time. Its price is listed at 600 Sim Points, but it cannot be purchased as a stand-alone item. However, it also apparently cannot be purchased as part of any sets. So it's in limbo right now, but I'm sure they will sort that out within a few days.
I have a feeling the Cow Plant may prove to be the game's biggest money-maker to date. Which is kind of a pity, because the more people who buy it, the more likely it is that they will leave the cool stuff out of the expansion packs, and force you to purchase them separately. But heck, I'm going to buy one!
The original Cow Plant was a bonus item in The Sims 2, which your Sims earned when they reached the top of the Natural Science career track. (I do miss some of the awesome career rewards - but really, none more than I miss the Cow Plant.)
In a world without Ambrosia or Young Again potion, the Cow Plant bestowed a form of immortality. It was also a fun way to kill other Sims. Really, what more could you ask from an object?
In The Sims 2, the Cow Plant had three stages: waiting to eat, digesting, and ready to milk.
Stage 1: Waiting To Eat
In this stage, the Cow Plant awaits its next victim. It is something like Audrey from "Little Shop of Horrors," a big round head which (as befits the name) is covered with cow spots. In this stage, the Cow Plant's trap is set: it dangles a tempting slice of birthday cake at any willing victim.
When a Sim reaches for the cake, the Cow Plant eats them. If you are controlling the Sim, you can direct them to "Eat cake." It is a little trickier with NPC Sims. I had my best luck by pausing the game and drawing up a fence to pen them in with the Cow Plant. They all gave in and tok the cake eventually.
Stage 2: Digesting
This takes a few days. When digestion is finished, the Cow Plant's udder becomes full and pendulous, and it produces another slice of cake.
Stage 3: Ready to Milk
Send your Sim out and command them to milk the Cow Plant. They will milk out a glass of fluid and drink it. And presto, it rolls back their life clock by five days.