How do I use Add-Type to load Microsoft.Web.Deployment?
PowerShell only allows a certain pre-defined set of assemblies to be loaded by their partial/simple name. You're going to need to load it via its fully qualified name, for example:
Add-Type -AssemblyName ('Microsoft.Web.Deployment, Version=7.1.0.0, ' + 'Culture=neutral, PublicKeyToken=31bf3856ad364e35')