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 System requirements. We will not repeat concepts and explanations made in previous parts of the documentation, here.
Please make sure, that you have installed your Visforms Subscription and that it has a miminum version of 3.2.1.
This will add a new field type (signature) to your Visforms installation, which you can use immediately with your forms. The new field type can be selected in the field configuration option "type" just like any other field type.