We have been asked repeatedly how to get the R4i Gold 3DS card working when encountering the error message #An error has occured. Press and hold the POWER button to turn the system off.

Well, the following error message means that the R4i Gold 3DS has not been installed with the latest firmware fix for the DSi or 3DS.
#An error has occured. Press and hold the POWER button to turn the system off. Please refer to the Nintendo Dsi Operations manual for details

Please note that as the R4i Gold 3DS is blocked by the DSi or 3DS, the installation of the latest firmware fix requires a DS Lite or a DSi with firmware lower than 1.4.2, at the point of writing.

The setup guide is available from here.

If you have doubt on how to check the firmware version, here's the guide on how to check firmware:

1. Click SETTING (with the Tool Icon) on the main menu
2. Check the right hand corner of the top screen which says VER 1.x