I recommend that you have a look at the code for the FDS example in our SDK, as well as the FDS documentation (both library and API), in order to understand how to use FDS for writing and reading data. A C string is basically just an array of type char, so all you need for storing it using FDS should be a pointer to the C string as well as its length.
A code snippet for writing the string "Hello, world!" is included in the Writing a record section of the Usage page of the Flash Data Storage library documentation.
