Hello and welcome to my User Tip
With Safari 5.1.7+ will disable older versions of Flash as a security precaution, all you need to do it update it or install it.
What is Flash?
Flash is a web browser plugin or extension that websites code their video and features in so it plays accross all web browsers and platforms. It doesn't work on iOS devices.
The user installs and maintains Flash on their computer, thus any website they visit that uses Flash then they can see the content or video as intended.
There are many web browser plugins, but the one used the most is Flash and thus many questions about installing it here, thus this User Tip
This User Tip is not intended to debate the politics of Flash or promote it's use. I'm well aware of the late Steve Jobs "war on Flash", many people who ask to install it so this User Tip is here to assist those who really need it so us volunteers can assist others with more important issues.
Install Flash Player
To install Flash, you click the orange link on this page. Bookmark this link and always use it to install Flash as there are those trying to trick you to install malware.
If you don't see the icon, it's downloaded to your downloads folder next to your trash can, double click that folder and doubleclick the Flash installer package.
10.8 users may have to head to System Preferences > Security > General and set the Downloads to "Anywhere" to accept software from the web that isn't signed by Apple. It can be turned back on when your done. A short cut is to right click or control key click on the installer or program and select "Open" to bypass Gatekeepers controls, once done it should always allow it to run. Always be careful your positive your installing the right software from a reliable source.
Keep Flash and other web browser plugins updated!!
Free checker, works for all browsers.
I advise you don't keep items pernamently in the Downloads folder, it should remain empty except when doing a recent download.
Malicious websites can initate a download simply by visting them, unknownst to you and leave a trojan wating in your downloads folder for you to click on and infect your machine.
You will likley need to enter your admin name and/or password to install, this is normal.
If it's not installing, uninstall Flash here first, then try to reinstall it again.
You might also have to manually uninstall any cache files Flash makes in the User account, look in the System Preferences > Flash for the cache delete button
Privacy issues and further troubleshooting
The Flash cache or "Flash cookies" should be cleared often as websites can read that information, it also might be corrupted.
Some browsers like Firefox have add-ons like Ghostery, Click&Clean etc., that perform this task automatically when the browser is quit.
If your still having trouble installing Flash
like your clicking the installer button and nothing happens...
#12 OnyX cache clearing routine has worked well for others in the past when Flash wouldn't install or isn't working properly for some reason, likely because of a corrupt cache file in OS X or perhaps a permissions issue.
Once OnyX has done it's thing, reboot and uninstall and reinstall Flash again to get it to work.
If your still having problems, perhaps it's something else and you need to run through the other Step by Step fixes or consult on the forums.
Like any software downloaded from the Internet, you need to trust the source.
I advise you not to manually install Flash by any other method besides via the link outlined above, so bookmark that link.
There is malware in the form of fake pop-up windows that looks like a real Flash update request and tries to trick you to click, enter your admin password or so forth to install a bogus Flash or other malware.
Here is a example of a Windows trojan here that mimicks the auto-update feature of Flash when it detects you are trying to view a video with a outdated version, then asks to update, it's actually from the website itself trying to install malware.
If any website or popup says you need to install/update Flash etc., it's probally wise to quit the browser completely and install Flash through known and trusted methods.
See this for example
BTW, I use Firefox for it's ability to display a window BEFORE a download starts to make sure I'm not being tricked.
For more about security issues with Mac's, read this