We already assume that you have Tor browser installed and running. If you don't have it installed, you can download it from here and follow the installation instructions.
Step 1: Download and install Flash Player for Mac.
This tutorial uses Adobe Flash Player v11.10 (Download link - here). If you are using Google Chrome, you might see the window below. Click on "Do you have a different browser or operating system?"
At the next window, choose the appropriate version of 'Mac OS X' as the operating system in Step 1 and 'Flash Player 11 for Mac' in Step 2.
After downloading the .dmg package, install it as usual by running the Adobe installer from the package.
Step 2: Copy the two required files
Once the installation completes, navigate to the following path:
Applications/Install Adobe Flash Player.app/Contents/Resources/Adobe Flash Player.pkg/Contents/Library.
From the Library folder copy the two files:
1. Flash Player.plugin.lzma
Paste these two files in the extensions folder of the Tor Browser (Firefox) at the following location: /Applications/TorBrowser_en-US.app/Library/Application Support/Firefox/Profiles/profile/extensions
Step 3: Restart Tor Browser and enable Add-ons
Restart the Tor Browser and open the Add-ons Manager at Tools->Add-ons and enable the Shockwave Flash add-on.
Now, the Tor Browser is ready to use and supports websites that require flash player.
Note: You may have to repeat Step 3 everytime the Tor Browser is started.