Why is the wow add-on timer buttons not showing the numbers

There are a few possible reasons why the addon does not work as it should:

1) The addon has to be properly installed (in the WoW\Interface\Addons folder) and has to be checked in the list of addons under the "Addons" button at the character selection screen.

2) The addon might be out of date, since patches regularly break the functionality of existing addons. Download the latest version of the addon and install it.
Also check if there's a red error message "Dependency Missing", which indicates it might need another addon installed and running to work.

3) Check if the configuration is done properly. Most addons have a console built-in, but you should also be able to access it via Escape -> Interface -> Addons. Make sure that it works properly and is set up as you want it.

4) Other addons might block the functionality. For instance, having another addon that replaces a frame this addon requires (such as button bars). Check on the webpage where you downloaded the addon if such is the case, there's usually also ways to fix it.

If none of the above fixes the issue, try to contact the author of the addon through the webpage where you downloaded it. Usually there's an attached forum or comment boxes - here you can also tell the author of problems you are having, and may get an answer.