how to use files in the msp430?

The same way you access files on any other platform - you need a File System.

There are plenty of commercial offerings.

A common free, open-source one is FatFs: http://elm-chan.org/fsw/ff/00index_e.html

Note that a file system respresents a significant overhead for a small microcontroller like the MSP430, but it has been done: https://www.google.co.uk/search?hl=en&q=FatFs+MSP430

The MSP has no storage device and no file system. So about what file are you talking and where is it?

To use files inside the MSP you'll need:

1) a storage device (e.g. an SD card, or maybe the MSPs internal main memory, which is very limited).
2) low-level drriver code that can do raw data reads from and writes to the storage device
3) a file system driver that interprets the raw data as file structures

So you need sort fo DOS/WINDOWS/LINUX or similar for the small MSP. Not impossible, but there is no off-the-shelf solution and the fact that you were asking indicates that this is (currently) beyond your capabilities to implement it yourself.

Outside MSP, the MSP is merely sending or receiving streamed data (through UART or oteh rcommunication channels) and the file handling is done on its coutnerpart, e.g. a frontend software on a PC.