It looks like you've found a small bug.
After installing Juicebox-Pro to Showkase-Standard, the label on the 'Site -> Customize Viewers' tab should change from just "Juicebox" to "Juicebox-Pro 1.5.1".
Thank you for reporting this issue.
I've contacted the developers who will hopefully fix this for the next release.
In the meantime, I think I've found the root of the problem (Showkase being unable to read the Juicebox version number from the 'juicebox.js' file under PHP 7.1.0 and above) and you might like to implement the following fix manually. I've tested it myself (under PHP 5.6.36 and 7.2.7) and it seems to work fine.
Open your site's 'showkase/admin/classes/themeset.php' file in a plain text editor and change line 229 from:
$docBlock = file_get_contents($sourcePath, false, NULL, -1, 50);
$docBlock = file_get_contents($sourcePath, false, NULL, 0, 50);
(The problem seems to have happened with a change in PHP 7.1.0 so if you are running a version of PHP prior to 7.1.0, you will not see the problem.)
Please note that the line number above refers to the current version of Showkase (v1.7.3).
I hope this helps to fix the problem until the developers are able to incorporate it into the next release.
Incidentally, when installing Juicebox-Pro into Showkase-Standard, you need only copy the Pro 'jbcore' folder to the 'admin/plugins/juicebox/master/' location (overwriting the existing Juicebox-Lite 'jbcore' folder). On next publishing your site, Showkase will automatically copy the 'jbcore' folder from the 'master' location to the 'viewers' location.