If you are using RepG 40 and have the correct Machine Type selected (one who's driver is "makerbot4gsailfish" as seen in Machine > Machine Information in RepG), then when you "Build to file", the file window which shows up gives you a choice of two output formats: .s3g and .x3g. Â You want .x3g (since you're using RepG 40).
Note that the file must be in the top level directory on the SD card. Â Subdirectories are not supported. Â And the SD card MUST be formatted as FAT16. Â (If it's >2GB, your operating system will likely try to format it as something other than FAT16.)