It's pretty straight forward to do what you want, but instead of looking at the ProcessBook Add-In templates you should just create an ActiveX control in Visual Studio. I've done so, and the add-in can be used in ProcessBook - but also Excel and other Office products supporting this.
Take the easy route and download the Microsoft All In One code framework, and base your project on the CSActiveX project which is included. It's basically a matter of designing your GUI inside a UserControl. As far as I can remember, the sample project from Microsoft installs the ActiveX in your computer when you compile (and removes upon clean). If you create an installer, keep in mind that you clean the project *before* you run the installer in your own computer or else there might be some trouble with registry settings.
*Also keep in mind - ProcessBook (and Office products) create an .EXD file (temporary file) you need to delete when ever you create a new version of your component. It's located inside the AppData folder for your user. It'll be recreated as soon as you open a ProcessBook file containing your component, or when you re-add your component to a PB display/Excel sheet.