The main page has a couple of paragraphs of instructions. Use CreateObject() to instantiate the CFC into a variable (you might even want to instantiate it in your Application.cfm/.cfc and store it in the application scope for easy access anywhere in your application).
As far as the usage, this paragraph sums it up well:
This component has two public methods, serializeJSON and deserializeJSON, which have method signatures similar to ColdFuison's built in Serialize/DeserializeJSON functions. The JSONUtil deserializeJSON method takes the same parameters as DeserializeJSON, JSONVar and strictMapping, and should produce the same results as that function. The JSONUtil serializeJSON method takes the var and serializeQueryByColumns parameters, just as ColdFusion's built in SerializeJSON function, however this method also takes an optional third parameter, strictMapping. If the strictMapping parameter is not provided or set to false the serialize method should produce the same results as ColdFusion's built in SerializeJSON function. If the Boolean strictMapping parameter is set to true, then the serialize method attempts to convert the ColdFusion data to a JSON string using underlying Java/SQL data types as opposed to using ColdFusion's implicit type conversion conventions.
-Carl V.