Is this what you have? http://www.createforless.com/Boye+Ya...pid112908.aspx
Lay your swatch flat, position the gauge so that 0 (zero) is at the left outside of the first stitch you are counting and move the slider over 10 stitches. The pointy part at the top will point the the number of stitches per inch.
Mine came with instructions, which of course I've misplaced and can't quote verbatim, but that's how it works. HTH
ETA: I had to find mine before I could answer your question. I don't really use it if I need an accurate count rather than close-enough and had to look at it before I was sure I remembered how to use it.