http://get.adobe.com/flashplayer/
Allow Plug-ins
Safari > Preferences > Security
Internet Plug-ins > "Allow plug-ins"
Enable it.
If this doesn't help try installing in Safe Mode.
Install Flash Player in Safe Mode.
Shut down the computer.
Press the power button. Immediately after you hear the startup sound, press and hold the Shift key.
The Shift key should be pressed as soon as possible after startup,but not before the startup sound.
Release the Shift key when you see the Apple logo on the screen. Startup will take longer than usual.
Login to your account. Launch Safari.
Enter this address in the Safari address bar manually.
http://get.adobe.com/flashplayer/
Follow the prompts. Install it.
Once done, quit Safari and restart the computer from in the menu bar.