When creating a meme, go to the top of the selected image. There is a small toolbar of options that include "spacing", "undo" (that is the 180 degree arrow), "draw", and "add image". (there is also that button with the sunglasses on it for small add-ons.)
For what you want to do, click the "add image" button and a window will pop up. There are the options that include where to add the image (either below it, inside it, next to it, etc), whether you want to upload an image from your computer, insert an image URL, or, below that, there is a search bar that reads: search memes. Type the name of the meme template you want to add in there and a list should appear of the meme templates that the site has that correspond to the name.
I hope this helps.