Memento Pro does not install any fonts on your system. Instead, it uses your computer's installed fonts.
Because of this, if you have Memento Pro installed on more than one computer, you will not necessarily see the same list of fonts on both computers.
If you would like to add a font to use in Memento Pro, simply install the font to your system font folder, then restart Memento Pro.
The font will now be available.
For specific instructions on how to add a font to your machine, please search the internet for "How do I install a font on (your operating system type)" - eg. How do I install a font on Mac 10.12?