[maltymike61]

I want to hook up a digital thermostat to a fridge I have in the garage for use as a fermentation chamber. I understand how it brings the temp down lower than the fridge's original thermostat is set for, but I've heard that to bring the temp up to around 68F for ales I need to also hook it up to a heater. Can someone explain to me how this works?

[JKL]

Is your temp controller a single stage or dual stage?  A single stage has a switch inside the case where you can control either the cooling stage or the heating stage.  A dual stage controller can control both at the same time.  Of course you need a heat source like a lamp. 
-J.K.L.

[tschmidlin]

You can't bring the temperature down below the thermostat of the fridge unless you disable it, the fridge will turn itself off when it gets there.  You don't need to for fermentation anyway though, the controller just gives you better control than the standard refrigerator thermostat and it lets you keep it warmer than most fridges allow.

If you want it warmer than ambient you need to add heat like JKL mentioned, but if the place you keep the fridge is warmer than your set temp it will be fine and you don't need to do anything.