first let me point out, the setting you are referring to will influence only images inside of PDFs you create with PDFCreator; if you just want to compress jpgs (and still have a jpg as output), you first need to change the output format to jpeg, which will then give you different settings:
To answer your original question: The setting JPEG-manual seems to not be working properly in the current version. I looked at the code and it sets QFactor
"-dQFactor=M (float from 0.0 to 1.0) Adobe's QFactor quality scale, which you may use in place of JPEGQ above. The QFactor scale is used by PostScript's DCTEncode filter but is nearly unheard-of elsewhere."
(from Ghostscript docs)
The default setting used by PDFCreator is 0.66, but somehow it isn't possible to enter any decimal (as you pointed out); we will look into it.
For the current version 0 is best quality and anything above 20 will probably result in unusable image quality.