ATmega 16u4 programming over usb

Hello,
I have a fresh ATmega 16U4-AU. I learned that it has a factory set bootloader that allows to upload hex files over USB using Atmel FLIP programmer. I setup a basic circuit with the USB connections(UVcc, Ugnd,D+,D-, Ucap etc). I have also attached an external 16Mhz crystal. I was unable to get the device detected by my computer. Then later on researching a bit, I came to know that I had to ground the HWB pin to activate the bootloader. Did that and still no good. Is there something that I'm missing. Is it necessary to connect all the Vcc pins to 5v. All the ground pins are internally shorted, but the Vcc pins are not so. So do I need to connect all vcc pins to 5v.

Thanks.