It can be obtained in all game modes, with the exception of the Zen Garden itself.
Once ten pieces of chocolate have been amassed, no more chocolate will appear.
How to obtain it
- Randomly from killing zombies.
- Occasionally eating a brain in I, Zombie.
- A streak of three in I, Zombie Endless.
- A streak of ten in Vasebreaker Endless.
- Sometimes after a Grave Buster eats a grave during a Night level, or during Versus Mode.
If the player wishes more than ten chocolates, they must open a game with nine chocolates. Then, before collecting another one, a different level should be opened. Collecting the chocolates in both levels will result in a number of chocolates higher than 10.
I'm not able to find a source which defines a specific drop-chance.
Presents drop very rarely and they may give the player Zen Garden plants, Chocolate or unlockable games.
Presents that appear after level 5-4 contain Zen Garden plants.
- Every three levels of I, Zombie Endless; however, this is not the case for the Nintendo DS version.
- Eating a brain in any I, Zombie level will sometimes drop one present.
- Randomly in Survival Levels, especially in Survival: Endless.
- Very rarely in an Adventure Mode level on and after Level 3-2, except for Level 3-10 where you can get, at most, four presents.
- Every ten levels of Vasebreaker Endless.
- Randomly removing graves with Grave Busters.
- Killing any zombie in any game mode.
- In Crazy Dave's Twiddydinkies, for $7,500, although this is only on the iPhone, iPod touch, and iPad version.
- For Day plants, Slot Machine and Wall-nut Bowling are the best.
- For Night plants, just replay Whack a Zombie over and over again, remove graves for coins, diamonds and sometimes presents.
- For Pool plants, play Zombie Nimble Zombie Quick, Bobsled Bonanza or Last Stand with Marigolds for farming. If you have unlocked Quick Play in the iPad version, you can play Level 3-10.
- For Fog plants, replay Level 4-10 (with the Zombie Yeti) or Survival: Fog (Hard) as there is no mini-game in the fog which has a high concentration of zombies.
- For Roof plants, Column Like You See 'Em is a good idea. You can also try Pogo Party.
- Level 5-10, Dr. Zomboss's Revenge, Survival: Endless, and I, Zombie levels drop every plant (except non-Zen Garden plants). Therefore, they are good ideas if you are not aiming for plants from a specific setting (although, Survival: Endless can actually be the best way to get plants, since it has the highest amount of zombies in the whole game).
- In the Xbox Live Arcade version of Plants vs. Zombies a present also drops in Level 2-1 containing Versus Mode and Co-op Mode.
- It is the rarest item that can be dropped from a zombie, dropping in a one-tenth rate.
- There are only 39 different Zen Garden plants of the 49 plants in total in Plants vs. Zombies.
- There is a glitch in the Nintendo DS version of Plants vs. Zombies where, at the end of a level, if the player clicks on a present with a Zen Garden plant in it, then touches the money bag, the player will get two plants, and both will be the exact same plant
- In the PlayStation Vita version of Plants vs. Zombies, zombies drop a noticeably higher amount of presents carrying Zen Garden plants.
- The PlayStation Vita version of Plants vs. Zombies has an occasional glitch where the player earns two Sprouts from a single present and both Sprouts are the same plant facing the same direction.
For your second question here is a link to a list with all differences in all available versions (could not find anything about differences in obtaining chocolate).
That are all information I found to your questions. If I find anything new I'll update.