To add a private font, click the Add button. The following dialog will appear:

Select a font file by using the Browse... button. Tick the boxes to specify whether your font is bold or italic or both, then specify a name for your font in the Name box. The mappings here can then be used in your documents with the font name you provided.
Note: Usually, you should have 4 font files for all variants for each font family: regular, bold, italic, and bold italic.
Note: You need to restart the service for changes to take effect. You can do that by checking Restart the service to apply changes.