WanaGo:
I could have a new project with no code, and I want to read the file name using the SDFAT library, and there would be no code to post.
If that was the case, you would have been wasting everyone's time by asking for help before you had made any attempt to solve the problem for yourself.
If this problem is a small part of a complex sketch then write a new sketch which demonstrates the problem in the simplest possible way. You will find this often helps you to make progress on the problem yourself since it forces you to test assumptions about which parts of your code are relevant to the problem.
In this case you want to open the directory (using SD.open()), access each file in turn (using File.openNextFile()) and get the file name (using File.name()).
If you know the file name includes a decimal number and want to know what the number is, you need to parse the number from the string. The C runtime library provides various functions such as atoi() to do that sort of thing.
This does not look terribly difficult and I would have thought you could at least have a stab at it before you give up and ask for help. Showing us your attempt would also show us which parts of this problem you have solved and which parts are causing you trouble, so the people trying to help you aren't left trying to solve the whole problem for you from scratch, which seems to be the situation at the moment.