Typically, you would open the project in Visual Studio and use the NuGet package manager to add the dependency, update the App.Config so that ASP.NET knows to use the Couchbase Session Provider and then build and deploy the output. When you add the NuGet package it will resolve and add any dependencies, such as Couchbase .NET SDK.
Couchbaseaspnet 1.2.3 only supports v1.3.X of the SDK, so you cannot use it with CouchbaseNetClient 2.2.6! However, there is a pre-release version of CouchbaseAspNet 2.0 available on NuGet. I would use newer version since the 1.3.X client has been mothballed. Please see this blog post.
This is true except you have to change the App.Config so that ASP.NET will know to use the custom session provider over the default in-memory ASP.NET one.