Program Files\Electronic Arts\The Sims 3\Game\Bin\Gameplay or C:\Program Files (x86)\Electronic Arts\The Sims 3\Game\Bin\Gameplay on 64 bit operating systems assuming you installed to the default location.
Lol!!! C:\ is the name of your main hard disk in your PC. So "C:\Program Files (x86)\Electronic Arts\The Sims 3\Game\Bin\Gameplay" means you start at the top, outside all your folders, and then find a folder called Program Files (or "Program Files (x86)") then inside that one called "Electronic Arts" and so on. You will need to use a utility that is always part of Windows, called Windows Explorer, which lets you move around in the folders and find files.
Please do not PM me with questions about modding. Please post in an appropriate forum and send me a link to the thread if you would like me to try and help.
Depending on your operating system you can usually click on your start menu (the little windows icon on the bottom left of your screen) and click on Computer and it shows you your hard disk drives C: is usually the first on the list. Double click on that and you should see your program files folder. If it's not there it is possible hide a folder accidentally in which case click on organize - folder and search options - view and select show hidden, files, folders and drives. From there you can right click on your program files folder and select properties and unhide it.