Download the offline installer here: http://fpdownload.macromedia.com/get/flashplayer/current/licensing/mac/install_flash_playe r_11_osx.dmg
If it won't download directly, right click and "Save as"
Mount the DMG and run the installer
If it won't run, go to: Mac HD/Library/Internet Plugins
Trash the Flash Player.plugin file from there
Go to: Mac HD/Library/Application Support/Adobe
Trash the Flash Player folder (the whole folder)
Empty the trash
Mount the DMG and run the installer again.