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?t=203&postdays=0&postorder=asc&start=30&sid=e60d31c04715f92a1d3414a71e3858b4.