After writing the classes that you want to use, the first step is to compile them into .dll files. These .dll files can then be referenced through the bottom part of the .NET script editor in Designer. Simply open the "References" tab, right click anywhere in the area, and then select "Add Reference". See below:
After clicking "Add Reference", another dialog will come up in which you will be able to browse for the .dll(s) you created earlier. To do this, simply click "Browse", locate the file, and then open it. It will then appear in the list of possible references to add. You must then select the reference and click the "Add" button in this dialog to fully add it into your form.
If everything worked correctly, you should now see the reference in the references tab shown above.