Are you seeing any specific error messages? If you could post them here, it will helps folks here resolve the issue faster.
That said, I'll try and provide some general troubleshooting steps. Hopefully they'll help.
You can download and install Flash Player from here: http://get.adobe.com/flashplayer
- Try using a different browser.
- Make sure that your user account has Admin privileges.
Some detailed troubleshooting instructions are here: