I just downloaded the latest pro version plg_jw_sigpro-v2.5.8_j1.5-2.5 and get this error

        It was not possible to copy the selected file.
        JInstaller::install: Failed to copy file: /home/wigit/public_html/tmp/install_503bc28a67dd7/jw_sigpro.php to /home/wigit/public_html/plugins/content/jw_sigpro.php

It's a permissions issue. Either use the free "Admin Tools" from akeebabackup.com to fix permissions or consult your hosting company. The first is obviously faster. :)

so...what folder/file permissions need to be fixed and what should the respective permissions be set to?

Check SIG PRO documentation please.

I ma pulling my hair out trying to figure this out. Everything look OK but obviously isn't. Can you take a look using the credentials i gave you for the 'Re: Upgrade ERROR' issue?

Right permissions on all joomla sites:
755 for folders
644 for files

As you can clearly see, the issue is not permissions. Please...i really want to get this paid plugin  working.

Pm me access to your backend as super admin please.