I've downloaded the Android SDK, and am running the SDK Manager. I've downloaded all of the packages, including Android SDK Platform tools (v 21.1.2). However, I don't have a folder called
platform-tools anywhere on my system.
Running OS-X 10.10.2
platform-tools folder is created as soon as you install SDK components using the download manager. It uses to be at the same level of the tools folder (there's no option to change that during the installlation process it should have been there).
The only suggestion I have is to use the manager again, remove it to then download components again. Use to be in the top of the list that shows in the SDK Manager window.
Anyway, I don't suggest you to install the sdk "too deep" into the directory tree. I had issues with that.
The library folder is usually hidden on a mac. Do this. Open Finder. Click Go. Hold the option key and library folder will appear. After that navigate to the android folder>SDK> and you will find platform-tools there. If that didn't help, you can find some more in-depth instructions here.