To the Space Wolves, attack is always the best form of defence. If charged, they will spring forward themselves and counter-attack the enemy. To represent this, unengaged Space Wolf and Fenrisian Wolf models from a unit that has been charged by the enemy must move up to 6" to get into base-to-base contact with the enemy. Treat the counter-attack as you would an assault move, so take terrain into account as normal.
Models that counter-attack do not receive the +1 attack bonus for charging but will be able to fight with their full complement of attacks. Long Fangs may not counter-charge because they are a bit old for that kind of thing! Models may counter-attack if the unit they are in was charged as part of a 'sweeping advance', in which case the move is made immediately after the unit that made the sweeping advance completes its move.