Thank you for writing.
In order to show the "+" button, you should set the NewItemVisbility property of the RadPageViewStripElement to a value different than Hidden, for example:
RadPageViewStripElement strip =
strip.NewItemVisibility = StripViewNewItemVisibility.Front;
You are correct that this is not covered by our documentation. We will address this case in a future release. I have updated your Telerik points for your feedback.
I hope this helps.
All the best,
the Telerik team
Q2’11 SP1 of RadControls for WinForms is available for download (see what's new); also available is the Q3'11 Roadmap for Telerik Windows Forms controls.