The website you want depends on the cartridge
Usually, for R4 Revolution (non-SDHC) - www.r4ds.com
and for R4 Ultra (SDHC) and R4i (DSi) - www.r4ultra.com
Once there, go to downloads and get the latest 'kernel' version for your language.
Unzip it and copy the 2 files and the 2 folders to your SD card.
R4 Websites
The website you want depends on the cartridge
Usually, for R4 Revolution (non-SDHC) - www.r4ds.com
and for R4 Ultra (SDHC) and R4i (DSi) - www.r4ultra.com
Once there, go to downloads and get the latest 'kernel' version for your language.
Unzip it and copy the 2 files and the 2 folders to your SD card.
Should be job done!