You'll have to figure out the name of the build action property. To find
this you will need a bit of probing code in your IWizard.ProjectItemFinishedGenerating
method first. Something like this:

private void ListProperties(ProjectItem projectItem)
{
 foreach (Property property in projectItem.Properties)
 {
   System.Diagnostics.Debug.WriteLine("Property name: " + property.Name);
   if (property.Value == null)
     System.Diagnostics.Debug.WriteLine("Property value: NULL");
   else
     System.Diagnostics.Debug.WriteLine(String.Format("Property value: {0}
({1})", property.Value, property.Value.GetType().FullName));
 }
}

That should get you a list of properties for a ProjectItem and their values
along with the type of value that each property expects. Armed with this
information, you should be able to set the property with something like this:

projectItem.Properties.Item("build_action_name_goes_here").Value = build_action_value_goes_here;

-----
Best Regards,
Dustin Campbell
Developer Express In