Currently the code editor only supports 1 code document, in this article you can learn how to use libraries in the Electronics Lab.
Using supported libraries
In the code editor, click Libraries to see the supported libraries. Click Include to add it to your design. This will add the necessary include code to your main sketch.
Using custom libraries
Since we don't allow you to upload external library files yet, the only current work-around to simulate third-party libraries is copy and paste the whole content of the library into the code sketch. It’s important to first copy all .h files (in the right order depending on the dependencies) and afterwards copy all .c and .cpp files, also don’t forget to remove all the include statements to the files you just have copied. The compiler will tell you all of this as errors if you make a mistake.
Keep in mind that we do not yet support all commands possible. For an overview of the capabilities of the simulator, check out this article: https://123d.circuits.io/blog/building-the-most-versatile-electronics-simulator-in-the-world