Slow green led flashing sound good to me (see LED messages here: https://dronin.readme.io/docs/blink-beep-codes). Your usb cable is probably the problem.
Thsi being said if you think the firmware is wrong, you can try to flash it manually: Start GCS, go to firmware tab, clic on rescue, unplug and replug the USB connection while the progress bar is still active. If the bootloader is detected,, clic on "open file...", select the BrainRE1 firware ( fw_brainre1.tlfw ), then flash firmware. If the firmware was successfully flashed, you can unplug and re-plug the usb and see your board in the GCS.
If it all fails, it could be a hardware problem, so just drop a mail to Martin Luessi (Brain tech support).