I have a bow that calls for a 60" amo string.
It currently has a 57.5 inch string on.

I just got a 60" amo sting at cabelas  and it is only 56" long    is that right.

Where can I get a good string that is 57.5"  

Thank you.

Is the string for a longbow or recurve?

recurve

I think your string will be fine, and it may stretch a bit also. Is it fast flight or B-50?

Depending on how tightly twisted the string is you can adjust it by twisting it. "Normally" Clockwise to lenghten, and counter clockwise to shorten, but keep an eye on the string while doing this see what it's doing. I know some left handed guys who make strings and they twist them in the opposite direction of a righty.