Probably he means Safari is already closed, so he can't quit it. At least that's what's happening with me, and apparently a lot of other people. The installer just mistakenly reports that Safari is running when it's not, and refuses to finish the installation.
Apparently the installer mistakenly reports that Safari is running when certain other applications that use the same WebKit framework, like Dashboard, for example, are running. There are several ways to get around this:
- Restart your Mac and relaunch the Flash Player installer before you start any other apps.
- Launch Activity Monitor and check to see if Dashboard is running. (It may be running under more than one process.) Select the Dashboard process and click the "x" button in the upper left corner of the window. Click "Quit" in the dialog. If this doesn't work (it takes a few seconds) try again and choose "Force Quit." You have to repeat this for each Dashboard process.
- Download and install the ClickToFlash browser extension. This extension allows you to avoid using Flash under most circumstances. It prevents Flash from loading automatically and, if possible, allows content to be loaded and displayed as HTML5 video. If you don't use Flash, you won't need to update it. It's an awful resource hog and security risk anyway.
Adobe could make this process a lot easier if they would make the installer a little more intelligent about reporting process names and providing a more convenient way to quit them if necessary. Apparently they don't think it's worth the trouble, so I guess you have to ask yourself if Flash is worth the trouble.