The article you have linked leaves out an important step it would seem. The action in the action studio requires the “Action Package” | “Options” to be setup correctly. In the attached action studio package I have set the Title to “Format String”, the Category to “String”, and Search Keywords to “Format String”. Setting these options means the action will be visible in the actions list and searchable.
Next you will need to also add the action packet to the packages list inside Finalbuilder. To do this go to “Tools” | “Package Manager …” then add your action package to the list. From this point you should be able search for and see a “Format String (.Net Style)” in your actions list.