Adding 2nd hard drive to windows 7

Hi,
im trying to add a WD 250GB hard drive as my second hard drive on a dell optiplex gx620

i have the power cord and the sata cord plugged in properly but when i turn my computer on it does'nt show up anywere not in disk management or my computer no were

What do it do so my computer will recognize the hard drive and i will be able to use it

Thanks