Did you try on a newly formatted SDHC card with nothing else on it?I tried to install RGF MCU
Try changing the name of Default.nds to _DS_MSHL.NDS but it doesn't work...
Make sure the only files you have on your SDHC card are the following from the R4SDHC folder in RGF MCU
_PATCH folder
_system_ folder
Games folder
TTMenu folder
_DS_MENU.DAT
_DS_MENU.INI
_DS_MENU.SYS
DEFAULT.nds
r4dldi.dat
The Games folder is necessary but it just makes life easier.
If this doesn't work please say what happens when you try to start your cart i.e. any warning messages or symbols etc.