This feature is part of the Visforms Subscription and not available in the free Visforms version..
We propose reading the documentation from the beginning, starting with the Introduction. We will not repeat concepts and explanations made in previous parts of the documentation, here.
Please make sure, that you have installed your Visforms Subcription.
What you get
This will add a new field type (calculation) to your Visforms installation, which is available at once. As any other field type, the new field type can be selected in the dropdown list "Type" in the field configuration.