As I can see in the picture, the battery pack has on each side a small black terminal which right now is covered with something for protection against short circuit. Just remove the black thing (probably shrinkwrap) and you can easily solder wires on these terminals. These terminals are allready electrically welded to the battery pack for ease of connection.
I had a Nick battery pack for a drill that ceased working after a fall. I opened it up and found that one of those terminals broke off the battery. I have tried to solder it back using a big solder iron and succeeded but it came loose again when I was using it. Probably this is another reason they are electrically spot welding those, so they don't melt when the battery pack is heating under load.