1) Are you creating the action as a .NET assembly (using COM Interop)? What language? If C# or VB.NET, to create the action component itself, you can start with the sample user action of the corresponding language (NETAction/VBNetAction under Samples\User Actions). Rename the project, modify the ProgId in the Test.cs/Test.vb file in the project as desired, etc.
2) After creating and compiling your user action component, you need to register it with COM. You can do this with the COM Register action (check the .NET assembly and Create a Codebase entry options).
3) Next, you need to create the VBP action which tells VBP about your component. In VBP, right-click in the Actions pane and choose Insert Action, fill in the fields (the only required fields are General | Name, Component | Action Type [COM Component], Component | ProgID [as defined in your project] and any properties for the action on the GUI tab), and OK the dialog.
http://www.visualbuild.com/Manual/actionspane.htm
4) Then you can insert a step for the new user action into a project and run it within a build.
The Samples\User Actions\User Actions.bld demonstrates automating the registration (with COM and VBP) and execution of a compiled user action (C# .NET Action / VB.NET Action sections).
http://www.visualbuild.com/Manual/userdefaction.htm