I believe you are doing things correctly with using the External Script function and I am glad to know that it shows under your username. The reason the script is not outputting is because a Code Block cannot be empty. Please consider writing a single line of code in the Code Block. For example, a HTML comment such as:
<!-- Linking jquery-1.7.1.js script -->
This will force CJT to output the template (i.e. external script) that is linked to your Code Block.