If you go to the following folder on your Mac: "{YourUserName}/Library/Application Support/obs-studio/plugins" do you see a file called "obs-plugin-countdown.plugin"?
If you don't then the plugin hasn't been installed. When installing it Mac OS will prevent it from installing as it isn't apple verified. You need to check
System Settings on your Mac and go to
Privacy & Security and check for something like what's show below and click on
Open Anyway
View attachment 93821
If you do are you able to check the logs for OBS (
Help>Log Files>View Current Log) and look for any log lines that contain "[obs-plugin-countdown]" something like:
"14:28:12.846: [obs-plugin-countdown] plugin loaded successfully (version 1.2.0)"
What logs do you see?