SdFat Library what is dir_t*?
dir_t * is a pointer to a variable of type dir_t.

I don't know the API to which you refer, but the format for passing it would be:

dir_t Bass;
char dName[13];
BrowseFile.dirEntry(&amp;Bass);
BrowseFile.dirName(&amp;Bass, dName);
Serial.println(dName);