I have resolved the issue I was having with the Property Page.

I had copied the Property Page from his demo project into my project folder. When I examined the property page in a text editor I saw that the definition for the LabelPlus1 control on the Property Page included his demo project name like this:
"Begin Proyecto1.LabelPlus LabelPlus1".

I simply had to change his project name to the name of my project.

Probably a rookie mistake but I've never encountered this before.