All the Waterwall tank models have a number of brass threaded outlets, which are moulded into the tank as it is made.  To use one of the tap outlets, drill out the plastic which has moulded over the outlet using a spade bit.  You can then screw in any tap fitting to suit the size of the outlet - all of the lower outlets are 1" NPT fittings, the knee-high outlet on the front of the tanks are 3/4" NPT fittings.

If you are linking tanks together, the Waterwall Joiner Kit includes a flexible hose fitting and a gate valve which suits the lower fittings.  See here for installation details on the Joiner Kit.