Hi Katie....Here's another explanation of how to use the kit from another walkthrough by Doug Radcliffe (the best WT in my estimation)!:
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"You'll use the fingerprint kit many times throughout the adventure - it's very simple to use. Open up your inventory by pressing the I button or by using the right mouse button and selecting the inventory icon. Once inside, pick up the fingerprint kit and use it on the object you wish to test for prints. In this case, pick up the kit and use it on the Holy Grail book. You'll be presented with a new screen with the book on the right and the kit on the left.
Pick up the fingerprint brush on the left and use it on the black powder, also in the kit. With the brush's end black from the powder, move it over the book on the right. Hold down the mouse button and sway your mouse from side to side in a brushing motion. Concentrate on one area for a few seconds and continue to move down until you've covered the entire object. If there's a print on the object, it should appear after a few strokes. If not, Grace (or later, Gabriel) will indicate that the object bears no prints.
If you do find a print, click the mouse on the scotch tape on the left. With a piece of tape in hand, click the mouse button over the fingerprint on the object. Next, click the tape (with the print on it) on the filing system on the left side of the fingerprint kit. This places the print in your inventory and can later be scanned into SIDNEY."
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Maybe it will do the trick. You should be able to get that print if you haven't missed anything. I'd try this first and then go back if you have to, OR, take a look at these
Walkthroughs Love, Witchen =O)