Hello and welcome to the forums.
Well, it could be a number of things.
You don't know how to do it.
You can't get your case open.
They won't fit in the case.
You don't have any SATA ports.
You don't have any SATA cables.
You don't have any extra power leads.
The SATA ports are not enabled.
You are trying to plug SATA II drives into an SATA I port.
The drives are both bad.......
In short, there could be a hundred reasons why you can't add them. Can you at least give use some clue as to what your problem is?