For anyone having the same issue but the above doesn't work, you may have extensions disabled.
In an older version of safari there was an option in the developer menu to disable extensions, I had previously clicked this option and forgot. I have later updated my OS/Safari and the option disappeared from the developer menu but it turns out the setting remained.
To change the setting you can:
Manually update the plist file.
Open the plist file in Xcode found at "~/Library/Containers/com.apple.Safari/Data/Library/Preferences/com.apple.Safari.plist"
and set the property "ExtensionsEnabled" to "YES" (mine was set to no).
OR
Use the Defaults Command
defaults write com.apple.Safari "ExtensionsEnabled" -bool "false"
I then re-opened safari and my extensions now showed up in the safari Settings
I hope this helps anyone having the same issue.
