you have to go through the "import"-button at the bottom-left on the Library-section. this will show you all the different drives available, including the external drives to which you might have saved your images (either CF or SD-cards these days) once you have chosen the pics and selected a directory at the section on the right, you click on "import". there are many parameters you can change, but, please, do read the instructions for that. importing pics is the only way LR will take notice of those pictures and store them in your catalogue
I believe the OP asks about Lightroom CC and Lightroom Mobile, your answer relates to Lightroom Classic. In Lightroom CC use 'File - Add Photos' or press the little plus button in the upper left corner. In Lightroom CC for Mobile you should have a small blue icon in the lower right corner with that plus.