Downloaded a zip File
When downloading the SuperCallouts on your machine it should initially download a .zip file which will contain the callouts (as png images). Double clicking on the .zip file will extract the callouts, which would place the callouts inside of several sub-folders on your machine (in the same folder the zip file was originlally).
To bring the callouts into Camtasia go to File>Import>Media, and the callouts will appear in the media bin, and from there can be placed on your timeline.
Download a libzip File
The .libzip format can only be opened on Camtasia Windows platform, however we can conver that file to a .zip file to extract the callouts.
To do so, right click on the .libzip file, then choose "rename" and change that extension from .libzip to .zip (just remove the lib part from the file).
Once that is done, follow the instructions above to import the callouts.