There’s a problem with PHP version. PHP should be at least 5.3, some functions of the plugin don’t work with 5.2.17. We’ve added an information message about PHP version in a recent release.

To update PHP go to: http://php-osx.liip.ch/

NOTE: Users do not need to install PDFTK with the PRO version of the plugin.

Formatting options and the select-box of PDF fields are only available when using the PRO version of the plugin or the free version with PDFtk through our server. Running PDFtk on your server will only provide a text field which requires you to enter the field ID as it appears in the uploaded PDF.

Indeed, the executable is installed here: /usr/local/bin/ so for MAMP and creating a symbolic link via
sudo ln -s /usr/local/bin/pdftk /usr/bin/pdftk
may be necessary.

While we are happy to continue providing standard support to install PDFtk and PRO2PDF on Linux, we are unable to provide any support for the Mac OSX – as it’s always missing a byte.

PDFtk development team only explains how to install the plugin on Linux here… http://www.pdflabs.com/docs/install-pdftk-on-redhat-or-centos/