First you're going to want to download the image of whatever tileset you want; feel free too google for some. Most creators don't mind you using them as long as you give credit, so make sure to do that! When you save the image to your computer, be sure to download it as a .png
So! Open Rpg Desktop, and in the top right tab you'll see resources. Click it. Once resources open you'll see you're in a tab labeled ''import' Click the resource type 'tilesets' since thats what you're adding to your engine.
Once imported, you're going to want to exit that tab and then go to the section underneath your current tilesets where your maps are visible. Right-click the map you want the tileset in and you'll see an option 'map properties.' You'll see that there is a list of current tilesets. Just click 'add' and whatever tilesets are in your resources, you'll have the option of adding!.