To connect a genuine Arduino UNO or a clone UNO to your PC USB port, you need to have an interface.
The interface built by rancidbacon for his tests is shown in the first picture. http://code.rancidbacon.com/ProjectLogArduinoUSB
The schematic for the interface is shown in the second picture.http://petrockblog.wordpress.com/2012/05/19/usb-keyboard-with-arduino-and-v-usb-library-an-example/
To be successful with this project, you will need to construct this circuit in some manner, either as shown here or as I showed in the previous section. While the construction is not complicated, errors will keep the V-USB software from working (said another way, the project will be a BIG flop and will not work.) Additionally, if you have a wiring accident, diode reversed, improper parts selection, or cold solder joints, the circuit is likely to not work. AT WORST, IMPROPER CONSTRUCTION AND WIRING COULD DAMAGE YOUR PC USB PORT, SYSTEMBOARD, OR CREATE OTHER SERIOUS AND EXPENSIVE DAMAGE.
DO NOT ATTEMPT TO BUILD THIS INTERFACE AND USE IT ON YOUR PC UNLESS YOU ARE CONFIDENT YOU CAN CONSTRUCT THE CIRCUIT PROPERLY AND HAVE THE ABILITY TO TEST YOUR INTERFACE BEFORE CONNECTING TO YOUR PC. IF YOU MOVE FORWARD WITH CONNECTING TO YOUR PC YOU ASSUME ALL RISKS ASSOCIATED WITH THIS PROJECT.