I have followed all the instruction mentioned at Creating Your First Safari Extension App. After running FirstSafariExtensionApp target, I have opened Safari preferences Extensions tab but I couldn't find My Extension in the left pane. I have Apple Developer membership and installed Mac Developer and Safari Extension Developer certificates in the KeyChain.
How to get my first safari extension app working?
Close your MacBook lid and goto sleep. Wake up in the morning and check Safari preferences extensions tab; You will find the extensions you are looking for. If you don't have time to sleep, try restarting the Safari or Xcode or Mac.