1. Extract slidepress.php to /wp-content/plugins/nextgen-gallery/xml/
  2. Create a gallery within NextGEN Gallery noting the gallery ID
  3. Create a gallery within SlidePress with the XML source type set to ?Default? and address field set to ?http://www.visser.com.au/wp-content/plugins/nextgen-gallery/xml/slidepress.php?gid=1?, replacing ?visser.com.au? with your own domain name and the value of the ?gid? attribute with the ID of your gallery within NextGEN Gallery.
  4. Save changes

Update: I removed fixed local file path dependence from the code, my next update will be to move the file outside of the NextGEN Gallery plugin directory making it upgrade safe.