If you have existing DLC, open the folder that contains the existing DLC.
In Studio, find the "media" folder in the project view.
RIght-click on "Media" and select "open in explorer".
You can now copy & paste the DLC contact you need for your project.
This will appear in the media files browser at the bottom of the screen.
AGK Studio is mainly a nice IDE for development. You can use the editor to create levels/static pages, but you'll need to create the code to actually use them.
The language is fairly simple, so use the tutorials and start simple. We all started our programming with no prior knowledge and if you get stuck, you will get help if you ask.