USB barcode scanner and Arduino
That seems to be a USB scanner. It mentions in the instructable that it won't work for USB: 


  While I started this project with the intent of "let's use that old CueCat" I couldn't ac-tually find mine, so I bought a "new" one off of ebay.  When purchasing one, it is extremely important that you get one of the original PS2 models.  Some of the ones for sale at places like library thing are USB and those WILL NOT WORK with this project


If your intention is to talk to the Arduino, you would either need to use one with USB host capabilities (don't know if they exist as I have not used one) or RS232 (mentioned as supported by the scanner - the serial connection here) which would be a lot easier, just use the Arduino UART connection.
So, I think if you get an RJ45 to 9-pin serial cable, plug the RJ45 into scanner and the 9-pin female into a 9-pin male (may need MAX232 depending on signal levels, may be able to pug straight into Arduino if it uses same logic levels) it should work. There should be plenty of Arduino tutorials on using UART/RS232 and (if needed) MAX232.