The answer lies in the build output:
2>C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1605,5): warning MSB3274: The primary reference "PaymillWrapper" could not be resolved because it was built against the ".NETFramework,Version=v4.5" framework. This is a higher version than the currently targeted framework ".NETFramework,Version=v4.0".
2>C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1605,5): warning MSB3268: The primary reference "PaymillWrapper" could not be resolved because it has an indirect dependency on the framework assembly "System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v4.0". To resolve this problem, either remove the reference "PaymillWrapper" or retarget your application to a framework version which contains "System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
In other words, the NuGet package uses a reference to .NET 4.5, so you can only use it on .NET 4.5+ projects.
You might want to ask the authors of Paymill Wrapper to see if they could publish a version which targets .NET 4 instead.
Note that the NuGet package page even states this:
Dependencies
- .NETFramework 4.5