You have two main errors in your code.
- As Brett said you're posting to the Index method, but you don't have Index method that supports POST verb. The easiest way to fix is to change Html.BeginForm() with Html.BeginForm("Test", "TestModel")
- You're using Html.DropDownListFor in a wrong way. You could pass only a value types there, because don't forget that the View will generate an HTML page. So instead of User in your Model you should have an UserID and in your View you should have @Html.DropDownListFor(x => x.UserID, @Model.listSelection). And finally in your Action you should query your data source to get the details for the user with this ID.
Hope this helps.