This feature was removed in AdBlock for Safari 2.64.
This is an advanced feature in two senses of the word. First, you'll need to enable AdBlock's advanced mode to see it in the options. Second, if you don't know what it is then you probably aren't using ClickToFlash (or its more generic cousin, ClickToPlugin) and you shouldn't need to use it.
What is it?
When Adobe Flash is not installed, AdBlock can interfere with the YouTube killer in the ClickToPlugin and ClickToFlash extensions on Safari. Since video ads are not displayed when the YouTube killer is in use, AdBlock doesn't need to block them. Enabling ClickToFlash compatibilitIy mode keeps AdBlock from making changes to the YouTube player that would prevent a video from playing.
To activate it, click the AdBlock button, select Options, and on the GENERAL tab select I'm an advanced user, show me advanced options, and then select Enable ClickToFlash compatibility mode.
Do I need it?
You only need to enable ClickToFlash compatibility mode if you use Safari with either the ClickToFlash or ClickToPlugin extension and you're having issues with videos on Youtube.com. If you're not using either of those extensions, don't enable it, because it will simply stop AdBlock from blocking YouTube video ads.
For more information, please refer to the ClickToPlugin help.
I don't see "Enable ClickToFlash compatibility mode" in AdBlock's options
If you don't see this option, it means that you have Safari content blocking enabled. We remove the setting from AdBlock's options when content blocking is enabled, because the two features are incompatible.