I'll assume you mean Flash. If you're sure you've installed the latest version of Flash, take each of the following steps that you haven't already tried. After each step, relaunch Safari and test. For a "missing plug-in" error, start with Step 5.
1. If you're only having trouble with YouTube videos, sign in to your YouTube account and load this page. You may see a link with the text "Leave the HTML5 Trial." If so, click that link.
2. If you're getting a "blocked plug-in" error, then from the menu bar select
▹ System Preferences… ▹ Flash Player ▹ Advanced
and click Check Now. Quit and relaunch your browser.
3. Triple-click the line below to select it:
/Library/Internet Plug-Ins
Right-click or
control-click the highlighted text and select
Services ▹ Open
from the contextual menu. A folder should open. Delete the following item, or anything with a similar name, if present:
Flash Player (failing).plugin
You may be prompted for your login password.
4. Re-download and reinstall Flash. Download it from this page:
Never download a Flash update from anywhere else.
5. If you get a "
missing plug-in" error, select
Safari
▹
Preferences...
▹
Security
from the Safari menu bar and check the box marked
Enable plug-ins.
6. Select
Safari ▹ Preferences... ▹ Extensions
from the Safari menu bar. If any extensions are installed, disable them.
7. In the Safari preference window, select
Privacy ▹ Remove All Website Data
Close the window. Then select
▹ System Preferences… ▹ Flash Player ▹ Advanced
and click Delete All. Close the preference pane.