How to open .cptl file.
Keep in mind that this will create a new .cptx file based on the template file so any changes you make will be saved as a regular project file.
To create a new template file, which I wouldn't recommend,...
Instead of creating new templates, create a Captivate Theme. While building your first of possibly many projects, modify the Master Slides, Object Styles, Table of Contents and Skin (playbar). These are the elements that make up a theme and you can easily apply newly created themes to just about any project made in Adobe Captivate. Themes are much easier to work with than Templates.
Do NOT use a cptl file, because the workflow has become very buggy. Especially if the cptl-file uses variables and advanced actions you'll get a lot of issues with projects derived from a cptl-file.
For design consistency use a Theme, not a cptl-file.
This is not only my recommendation, and I have used those template files in the past, but never since several versions when I have seen all the problems. Ask othe Captivate experts and you'll often get the same tips.
An atlernative for a cptl file is to create a normal cptx file and block the file for editing. You can use a duplicate of the project to create new files. Much safer and without the issues of cptl files. That cptx project should have a solid custom theme with theme fonts, theme colors, object styles, master slides, skin and eventually Recording defaults.