Before you can use the FinalBuilder templates, they must first be added to TFS. The first step is to add the FinalBuilder templates (from <FinalBuilderInstallDir>\TFS Templates) to your Team Project's source control. I suggest adding it to <Your Team Project>\BuildProcessTemplates, alonside the TFS-provided templates.
Now, in Team Explorer, under your team project right-click on [Builds] and choose [New Build Definition...]. Go to the Process section. Under "Build process template" choose "Show details". The drop-down list allows you to choose from any of the templates that have been added. Because this is the first time you've used the FinalBuilder template it will not be in the list. To add it, click [New...]
Choose [Select an existing XAML file] then browse to the location in source control where you added the FinalBuilder template. Press [OK] until you are back at the Process section.
The FinalBuilder project template should now be selected in the drop down list. The next time you create a new build definition in this team project the template will be in the list.