What version of the Mac operating system do you have installed? The veresion of Adobe Flash Player that Pandora wants may not be compatible with older versions of the operating system. You can check the version of your OS by clicking on the Apple shaped icon at the top left of your screen and choosing 'About this Mac'. The second line of type will give you a version number.
Here's information from the Adobe support page at http://helpx.adobe.com/flash-player/kb/installation-problems-flash-player-mac.html
1. Verify that your computer meets the system requirements for Flash Player
Compare your computer system to the Flash Player system requirements. If your system doesn't meet the requirements, don't continue to step 2. See Archived Flash Player versions.
2. Determine whether an older Flash Player version is already installed on your Mac
See the Flash Player information displayed above, and check if you have the latest available Flash Player version.
The latest Flash Player version available for Mac OS:
Mac OS X 10.6, 10.7 (Snow Leopard, Lion): 18.104.22.168
Mac OS X 10.4, 10.5 (Tiger, Leopard): 10.3
If Flash Player is not installed or you don't have the latest version, download and install the latest Flash Player version.
Important: If you have Mac OS version 10.3 or earlier, or if your Mac does not have an Intel processor, do not download the latest Flash Player version. See Archived Flash Player versions.
Repeat step 1 and step 2 for each browser that you use.
3. Download and install the latest Flash Player version
Flash Player for Safari
Flash Player for Firefox
Chrome with Flash Player