Hi,
You are messing a few things together. Workflows do not execute plugins, but a custom workflow activity which also can be a .net library. They share a common Event Framework, but they are generally separate things (e.g. workflows can be set up to run asynchronously).
Anyway, your need can be accomplished without any code and plugin. Do you need a plugin for any reason? If you only need to dynamically assign a newly created case to a specific user, you can create a workflow that will run on the case creation and will only perform one operation "Assign record" and then you choose who should be the owner.
However, if you really need to use plugins, not workflows for any reason, please take a look at the samples in the SDK: http://msdn.microsoft.com/pl-pl/library/bb955365(en-us).aspx (how to create a plugin), http://msdn.microsoft.com/en-us/library/bb959372.aspx (how to assign a record to a user by using the AssignRequest request).
Let us know whether that helps or if you need further assistance.
Regards,
Kuba Skalbania
