We have not tested our plugin with MAMP. It’s likely that PDFtk is not started with just a PDFtk command in console, but another path or something very individual.
PDFtk development teams only explains how to install the plugin on Linux: http://www.pdflabs.com/docs/install-pdftk-on-redhat-or-centos/.
The executable is installed here: /usr/local/bin/ which is probably not included into $PATH environment variable on MAMP so, for MAMP creating a symbolic link via sudo ln -s /usr/local/bin/pdftk /usr/bin/pdftk may be necessary.
We do not offer support for MAMP, try contacting MAMP for assistance with making PDFtk work on your local machine.
Otherwise, we remain at your service.