To use your first question from the Community Question Library, start by going to any question's page and clicking the Download button.
This will download a .zip file, but you should not extract the file contents. Instead, open Lighthouse Studio and find the Question Library area. This can be found in Compose > Question Library or by clicking the book icon in the main menu or Write Questionnaire window.
Within this new window, locate and press the Import... button to import the .zip file downloaded earlier. If imported successfully, the new question should appear in the Question Library window.
When you next go to add a new question to a survey, you should see a Library tab.
In that tab, you should find a dropdown with two library options. Sawtooth Software Library is a collection of built-in questions offered from Sawtooth Software. My Library is your own local question library and where you'll find the question you have just imported. Selecting the imported question here will add a copy of it to your survey.
Understand that while many questions from the Community Question Library work out-of-the-box, some have additional instructions or optional configurations relating to getting the question to work for your specific survey. Please consult your downloaded question's Community Question Library page for additional information on how to use the question.