Design-Editor has fonts pre-loaded into the system, however you also have the option to add in custom fonts directly from your computer, through Google Fonts and through Typekit.
Double click into your text element and open the Design Panel.
Select the font drop-down and click the ADD FONTS button at the very bottom
You may been to make the Design Panel longer to see the Add Fonts button
A window will appear with three tabs to choose from: Add Google Fonts, Upload Your Fonts, and Connect Typekit Fonts
Add Google Fonts
Use the Search field to look up your font, then select it from the list. It will appear in the right hand section under "Your Selected Fonts"
Click OK to save.
Upload Your Fonts
Click the Upload Font Files button and add the fonts from your computer.
Once added, your selected fonts will appear on the right. Click OK to save.
Keep in mind that not all fonts are supported by all browsers.
Connect Typekit Fonts
Generate your Typekit token and paste it into the field provided. If you do not know how to generate the token, first login to your Typekit account then click the "Generate your Typekit token" link. It will be generated for you, and you can copy and paste it in.
Once the token has been submitted, you will see a dropdown menu where you can select your specific kit to add. Once added, your selected fonts will appear on the right. Click OK to save.