@alexiskraft11
Do you have an external hard drive you can install Yosemite onto? Here's how you make a drive bootable and install Yosemite onto it (the drive will be wiped so make sure there's nothing on it you want to keep):
- Plug in your external drive and open Disk Utility (Applications > Utilities > Disk Utility). Click on the drive in the left side window then click on Partition.
- You want it to be one partition, GUID Partition Map so it is bootable. Call the partition whatever you want and make sure the format is OS X Extended (Journaled). DON'T click on your internal Macintosh HD, that is the one you are currently using.
- Click Apply
- When it's finished close Disk Utility then open the App Store.
- Go to your Purchased tab and find OS X Yosemite.
- Click Download. When it's finished downloading, the install window should appear. Make sure you select the new drive you have just formatted to install onto and then follow the instructions.
- Your Mac should install and automatically boot into the external drive. You can install Sims 3 onto the drive in Yosemite.
- VERY IMPORTANT - when you want to choose which drive to boot into, the new one you have created or your Mac's normal internal one, hold down the ALT key when you start up your Mac. Keep it held down until the bootable drives show in the middle of the screen. You can now tab to the one you want to boot up and hit Return.
Apple have written a help guide here which may also help - https://support.apple.com/en-gb/HT202796
