Add a new font path to Scribus:
Open Scribus. Before opening or making a new file, go to Settings > Fonts and select the Additional Paths tab. (If you need to add an invisible folder, like .fonts, just type it in.)

Right-click on a guide to open the Manage Guides panel.

To make an image fit inside its frame:
Right-click on the image, and go to Properties (or press F2). Under Image, select "Scale to Frame Size".