I have coded the initial support for SVGFonts and the SVG Fonts Editor as part of my GSoC last year. So I can help you here.
"there is no way to save the font"
Actually there is a way. Save the SVG file. The fonts will be embedded in the document. For instance, this svg file can be imported into Fontforge in order to convert it to a truetype font.
"I was expecting the font to show in the list of my fonts"
well... currently we dont have integration of the svg fonts with the canvas. We are waiting libpango guys to implement proper support for the user-fonts feature in the lib. That's what currently blocking us. So, nowadays the only use of svg fonts in Inkscape is in the font design workflow, by exporting SVG files with fonts embedded and importing them into Fontforge.
There is a concept that has to be understood while using the SVG Font Editor. SVG Fonts have mapping of chunks of svg drawings to characteres. When a certain char is used in a string, its respective glyph is rendered. If no glyph is declared for a certain charactere, then there is a default "missing glyph" that is rendered. You can set the drawing that defined this missing glyph also. This is done by clicking on the Missing Glyph: From Selection... button at the top of the dialog.
so, an example of workflow would be:
1) click new font. Select it. You can rename it if you wish. You will see a set of black squares in the text preview area. This is the preview text being rendered. It only uses the default missing glyp (which is defined as a black square) because no specific glyph was defined yet.
2) draw something (your desired missing glyph)
3) click Missing Glyph: From selection...
4) draw a glyph for the "a" charactere (charactere matching is case sensitive)
5) on the glyphs tab click Add glyph
6) type "a" in the "Matching String" column (at the moment the glyph-name is not important. It still needs to be implemented, so at the moment it is irrelevant)
7) with the row selected, click "Get curves from selection..."
8) now you will probably see the "a" glyph in the preview rendering. I'm assuming the the preview text is "Sample Text", which contains the "a" charactere. Yopu can freely edit the preview text in order to test-drive the font rendering.
You can repeat steps 4 through 7 for every other glyph you wish to add to your font.
Felipe Sanches
a.k.a. JucaBlues at #inkscape irc.freenode.net