Hello,
The .dat files can be multiple things in Curry, the same extension is used for averaged ERP and for the positions of the electrodes. The extension .rs3 is only for 3D positions, so you are maybe trying to import the wrong type of data into Brainstorm.
If you are using the menu “Review raw file”, it’s probably because you are trying to read the continuous recordings from the Neuroscan software. In this case, you should be selecting the file format “EEG: Neuroscan”, and select a .cnt file.
Francois