The only method of adding/uploading your own custom fonts is through the dashboard, you will not find the option to do so through the editors. For more information visit: Choosing Google Fonts or Upload your Own Fonts.
In the settings tab of the dashboard, in the fonts link area, you can add pre-existing fonts Google font through the Google Fonts tab, simply selecting the new ones you want to add and then clicking add. For custom fonts, you select Own Fonts tab at the top which will then allow you to upload your own downloaded font and add it to the list.
Then when you have done this, you can go through the editor and use the font you have uploaded freely.