As you say they are BELOW the tank on the left of the bike. Depending on which year bike you have either a DIN plug or a USB plug. If you have the USB plug you can just plug a lead from the charger in to the USB, no need to remove the tank or wire anything to the battery.
If you don't have the USB port and you want to install the item you bought you can get the pos on the bar just behind the tank under the seat and the negative can go on any bolt that makes contact with the frame (i used one of the middle seat frame bolts).
Another alternative is to plug it into the GPS connector, you need the connector from BMW to do that, search for the GPS thread in electronics... This plug is tucked a little bit more away, it is forward of the DIN/USB plug
good luck