Warren,
In PlanetPress Design, look in the for errors in the PlanetPress Talk messages Window.
This type of error could be caused by image(s) used in the form (ex: pdf file).
You could removed the images from the form, if any are used, then do a preview.
Also from the Objectif Lune KB:
http://www.objectiflune.com/OL/en-CA/Services/Support/KBArticle.aspx?ERR=p0054 Error P0054 - PlanetPress Design
Description : There was an error in the convertion to PDF.
Problem : While attempting to preview the document, an error occured.
Cause : An error occured while trying to convert the file into PDF format to preview it. The Postscript interpreter should be checked for any conversion error.
Solution : Look for another error message in the PostScript interpreter's logs to accurately pinpoint the source of the error.
Regards,