My impression is that the easiest way to show the value is by creating a new Value object, attaching it to your dataset, and then showing its value. You can hide this Value object if you want to.
It appears not possible reading from calculated DataSets in ProcessBook directly. To get the values in VBA you need to assign a ProcessBook symbol, as suggested by Marcos, and read from the symbol.
An alternative approach could be using e.g. the IPICalculation interface of PI SDK to retrieve calculated data directly from the PI Server.
This is an old post, but you can use:
ThisDisplay.Datasets("RATE OF CHANGE").DataPoints(1).Symbol.GetPIValue
Where 1 is pointing to the datapoint you want to reference.