From within your course, click on the Content tab located in the navbar.

From within the content area of your course find the module you would like to add the Survey to. For this example we will be adding the survey to the Survey module. Click the gray Existing Activities button and select Surveys from the dropdown

From within the Add Activity box that populates, click on the survey you would like to add to the course. From this box you can also create a new survey or manage the surveys you have already created.

Once clicked, the survey you added will appear in the module you added it to.