Hi Thomas,
The property externalFunctions belongs to the system of unit operations. Any external function has to be registered there.
In the example, a SingleGRM object is used. This object represents a single GeneralRateModel within a ModelSystem (for convenience). When the externalFunctions property on the SingleGRM is used, it actually accesses the property on the ModelSystem part of the SingleGRM.
Long story short: Please use mSys.externalFunctions instead (if your ModelSystem object is called mSys).