The version of the API you are referring to is an older one and you don’t need to use it since the latest GroupDocs.Signature for .NET is a UI independent API, with a lot more features that can be integrated in your web based app as well. Kindly follow the following link to download the API. Moreover I would suggest you to download the sample github project which contains the examples of the features supported by the API. Also have a look at the documentation of the API, specially the features overview for a better understanding of the API.
Let us know in case you find something missing. Looking forward to your response.