The voltage divider was added in Rev6. You can see the diagram and the resistor values in the MCU section above the main symbol in the 6c schematic: http://support.sodaq.com/wp-content/uploads/2018/02/explorer_schematic_rev6c_868.pdf
The AIN for the reading voltage is attached to PB05. This is not in the current board files but will be released in the next version (v1.6.20). The pin will have a defined label BAT_VOLT.
If you need it before then you can get a copy of v1.6.20 from the test release here:
Please note that the test release versions will change and this can cause issues with the Arduino IDE's cached copies and their checksums. This can always be fixed by cleaning out the 'staging' folder that Arduino creates (the default location on the Windows platform is in /AppData/Local/Arduino15).