If working with bitmap fonts, you need two files - the .png bitmap *AND* an accompanying text file named <fontName> + subimages.txt - if using a font-tool like Fontforge or BitmapFontMaker it'll generate the accompanying text file for you. This text file follows the format: ascii ID:startX:startY:endX:endY newline and for each character it needs such an entry. Doing that yourself, whilst possible is very tedious.
Luckily, with AppGameKit (for windows at least) a bitmap font creator is included.
Next up, in AppGameKit you import the font as per usual - loadImage(fontID, "<fontName>.png") - and that is pretty much it. It'll auto-load the subImages file. To use the font, just reference it when creating text, like so:
CreateText(textID, "hello, world!")
SetTextFontImage(textID, fontID)
SetTextSize(textID, 24)
SetTextPosition(textID, 50, 50)
SetTextVisible(textID, 1)
...and on next call to sync() it'll draw your text.
Though using bitmap fonts is a bit passe these days - your much better off using true-type fonts, which is much the same procedure in code, however you do not then need the subImages file. Just copy the font to your media folder, and import using loadFont(fontID, "<fontName>.ttf")