Rosegarden is supplied with a single scalable notation font, the Feta font developed by the LilyPond project. Feta is an extremely high quality font that should be suitable for nearly all classical work.
Rosegarden also includes support as standard for a number of other notation fonts. You will need to provide the fonts themselves in TrueType (.ttf) or PostScript Type-1 (.pfb, .pfa) format, and you will need the relevant permissions to install them to the same place as Rosegarden was installed. To install a supported notation font, locate the share directory for the Rosegarden installation (usually a subdirectory of the KDE desktop installation tree, such as /opt/kde/share/apps/rosegarden). You will find that this directory has a subdirectory called "fonts": copy the font files into there, and restart Rosegarden. If your font is supported, it should now appear on the font menu in the notation editor.
Complete or partial support is currently provided for the following fonts: Fughetta, by Blake Hodgetts; Petrucci, the "original" Finale font; Maestro, the "new" Finale font; Opus, the Sibelius font; Inkpen, the Sibelius jazz font; Sonata from Adobe; Steinberg, from the Cubase sequencer; and Xinfonia. (Most of the above font names are registered trademarks of the respective companies.) THE ROSEGARDEN TEAM MAKES NO REPRESENTATION AS TO THE LEGAL STATUS OF ANY USE OF THESE FONTS WITH ROSEGARDEN. It is your responsibility to ensure you are in compliance with the licence (if any) under which a font was provided to you, before attempting to use it. If you are not sure whether you have the right to use a font, don't: apart from anything else, the standard Feta font is better than most of these anyway.
It is also possible to provide new mapping files so as to use new fonts. For some fonts that have similar mappings or metrics to fonts in the above list, this may be as simple as editing the name of the font in the mapping file. See Customising Rosegarden for more information.