Depends where you do not see it. If you do not see it in the BIOS, you have a hardware problem: bad drive, no power, bad data cable, bad motherboard SATA interface.
If you see it in the BIOS but not the OS, the usual problem is that it is not formatted. You need to format it and assign it a drive letter. Go to the Menu bar|Help and put something like "format drive" in the dialog box. Then follow the instructions. You should end up in a place called "Disk Management".
Keep in mind that a bad drive may show up in the BIOS but not Disk Management.