If you've created a new website and don't see a font available to use you know is in use on other sites in your account, or you know has been added to the account, you can activate it in the Fonts Panel.

First, open the Fonts Panel by pressing Shift + F on your keyboard, by opening the Text Edit panel and clicking "Add New Font", or by going to Window > Font Manager. In the upper right hand side, click into Website Fonts and select Default Fonts.

Once you do that, you'll see all the fonts available on your account. Find the font you want to add to your site and click on it.

Toggle back to Website Fonts in the upper left, leaving this font-specific screen open.

Once you're back to Website, you'll see that the font in inactive:

Click on Inactive to toggle it on to Active, which will now add the font to your site.

Lastly, scroll down and make sure you have all the font weights available selected, otherwise they won't work on the live site.