The instructions for Upgrading to Pro should work fine so please try again.
Here's an expanded set of instructions:
(1) Extract the juicebox_pro_1.5.1.zip file to your hard drive and you'll find the Pro jbcore folder in this location: juicebox_pro_1.5.1/web/jbcore (Please double-check that you are using the Juicebox-Pro zip file and not the Juicebox-Lite zip file.)
(2) Navigate to the showkase/admin/plugins/juicebox/master/ directory on your web server and delete the existing jbcore folder there (showkase/admin/plugins/juicebox/master/jbcore). (This saves you from having to check if your FTP program is set to overwrite existing files in Step #3 below.)
(3) Upload the jbcore folder from Step #1 above (juicebox_pro_1.5.1/web/jbcore) to the showkase/admin/plugins/juicebox/master/ directory on your web server.
(4) Republish your site (click the 'Publish' button). (Juicebox galleries within a Showkase site are not updated until the site is republished.)
(5) Clear your browser's cache before reloading your gallery pages. (This will ensure that your browser is not still using older cached files from prior to the republishing.)
Incidentally, if you are not already using the current version of Showkase (v1.7.4), then please upgrade. There was a bug (which was fixed in v1.7.4) which prevented the correct version of Juicebox being displayed in 'Site -> Customize Viewers' and 'Site -> Change Viewers'. (This did not affect any functionality but could be confusing.)
Full instructions can be found on the Upgrading Showkase support page.
For reference, a list of changes between version can be found on the Version History page.
I hope this helps.