The glyphs for zodiacal signs, planets and aspects are stored in a font that is automatically installed by the installation program resp. Linux package. Nevertheless sometimes manual installation is necessary.
The font file name is Saravali.ttf, the file is usually located in the subdirectory fonts. The exact location is described in the Directory Layout chapter.
Please note that in the symbol font is not contained in the standard maitreya package in the Debian repositories. Is is contained in the extra package fonts-maitreya8.
Manual font installation is platform dependent.
Double click on the font in the Explorer, click on the installation button in the next dialog.
Open a file manager (Nautilus, Dolphin, Caja etc.), right click the font file and install it.
Alternativly, the font file can be copied to the directory $HOME/.fonts.
Right click on the font file in the Finder and install it.