Hi Wade,
You should be looking at motion graphics and stock footage files. These are essentially video files, so will work in an editing package like Camtasia.
aep files are After Effects project files and require Adobe After Effects to open them. Once you render the file, it’s video and you can import it into Camtasia. But you need After Effects to render it.
You can actually download a 30 day demo of After Effects from Adobe if you want to use the .aep file that you have purchased.