Hii,
In AVR microcontrollers, you have to set the fuse while programming the uc for enabling it to use external crystal (otherwise it will use internal 1Mhz crystal by default).
The Fuse LowByte is having 4 bits (CKSEL3..CKSEL0) for selecting the clock source, which can be set to to match your application. (check the 'memory programming' section of the datasheet of the AVR uc which you are using)
In AVR Studio, select the programming dialog for STK500, there you'll find FUSE tab. In that page, at SUT_CKSEL label, you can select the option you want for the external clock source