The filesystem can be directly accessed through "/dev/fs".
There are several IOS functions for FS like Delete/Create/List/Rename/.. .
Files can be directly opened with IOS_Open("/tmp/data.bin",1);
The root folder looks like that:
For a more detailed example of how the file system structure can look, see http://www.openwii.org/forums/viewtopic.php?p=1241#1241.