I just posted this in mysimBrenda's thread but thought it may help to have a dedicated thread with instructions as more and more Mac users are realising that the game only truly works properly under Windows. Snapdragon has also provided a fantastic link to an external site detailing how to do the installation:
You will need a retail copy of Windows 7 or Windows 8 to install into Boot Camp and a blank CD or USB stick (Fat 32 formatted) to save the Support Software onto.
Boot Camp partition size:
Lots of people ask this but it's up to you to work out how much space you should give the Boot Camp partition as everyone's games are different. As a general rule go with the following:
Add up the following:
1. Base game requirement size + All EPs and SPs you have + 20Gb for Windows OS
2. The size of your current Sims 3 folder in OS X. This is your user data. To find the size: Documents > Electronic Arts then select The Sims 3 folder, hold down CMD and press i
3. And then add 10Gb on top so the Boot Camp partition isn't full. Or 20Gb/30Gb if you have the space for any future CC/downloads/big save files/EPs/SPs.
4. Want to install CAW? Then add another 5Gb or so.
Copying over your user data from OS X:
Snapdragon has posted about what you should and shouldn't copy over, and why, here:
You can access your data from the Windows side - no need for an external drive to copy and paste - by going to My Computer > Macintosh HD > Users then navigating to The Sims 3 folder in your Documents folder. You can then copy and paste data or drag it into the relevant folder in Windows.
Make sure any peripheral devices connected to your Mac are unplugged (thanks suepea!)
Applications > Utilities > Boot Camp Assistant
I strongly recommend you print off the Installation & Setup Guide:
Make sure both boxes are ticked:
If you already have a Boot Camp partition installed and want to get rid of it and start again you will get this page instead:
most important bit (you can always reopen Boot Camp Assistant and re-download these if you bypass this step by accident) :
Downloading the Apple support software:
After it has finished the download it will then ask you how big a partition you want for Windows:
Chose the partition size and leave it to do it's stuff. When it boots into Windows you will need to reformat the Boot Camp Partition as NTFS as your Mac has formatted it as Fat 32.
After it does what it needs to it will restart into Windows. It is at this point that you install the Package file you burnt to disk or saved onto a USB stick. You do not need to download anything else from anywhere online to get the installation to work properly.
Once installed properly you will see this in a Finder window in OS X:
And similar under My Computer in Windows. You can read to the Boot Camp partition but not write to it, and vice versa to the Macintosh HD when you are in Windows (great for accessing screenshots you took in game in Windows and editing them in iPhoto).
Hope this helps a bit
*04.09.14* adding in the link to Apple's Boot Camp page for further help on installing from ISOs -