It is automatically generated when you start the game in 1.5.2 or below, but for 1.6 and up, tlthe .jar is stored in the version itself.
Private Mod Note
Rollback Post to RevisionRollBack
DO NOT OPEN THAT SPOILER!!!
If you are reading this then you you totally ignored what I said about not opening that spoiler three spoilers ago.. Don't you remember? I said "DO NOT OPEN THAT SPOILER!!! But no, you were a rebel and did it anyway!
so I drop it in my folder with all the versions?!?!:unsure:
No; there is a folder inside your .minecraft folder named "versions" and inside of that you can find folders for each version, such as "1.8.3" for 1.8.3, "1.5.2" for 1.5.2 (note that even older versions are now stored like this, contrary to the post about 1.5.2 and older versions still using the bin folder - only if you use the long-obsolete 1.5.2 and earlier launcher, which should not be required and I don't even think you can log in with it anymore).
Inside of those folders you'll find a jar file named after the version, such as 1.8.3.jar, which is the modern equivalent of Minecraft.jar, and a json file named similarly; if you want to directly mod the jar you have to do some extra steps compared to what you did before 1.6 (the launcher will try to redownload a file it thinks is corrupted or out of date, thus you have to rename it to a file name not matching any on Mojang's servers (the launcher ignores the HTTP 404 error if there is a copy on your computer), but in a way things are easier since you can use multiple profiles and game directories (where saves are actually saved, so you can save modded or other version worlds separately (really bad things can happen even with vanilla if you load worlds into the wrong versions); for example, I use my main Minecraft profile for my regular Survival world, and a "testing" profile for other stuff, with a different save location; you can change this in the profile editor).
However, you can also use the Optifine installer; just make sure you've run the version you want to install it on first (they are actually the same; the installer is inside the Optifine jar file, which can also be placed in the mods folder if you have Forge; I just do it the way mentioned above since that's how I've always installed it, along with my own mods, the exception when I used Forge, then I installed Optifine using this method (note - only attempt if Forge doesn't like Optifine, mainly with older versions; if you use the right version it should work) and even my own mods, placed inside a jar file and added as a library).
Note also that this thread was started in 2013; it is also best to ask stuff like this in the modded client support subforum.