Hi Ross,
Please have a look at pages 16-18 of this document:
http://www.pioneerb1.com/wp-content/uploads/2012/04/How-to-work-with-Crystal-Reports-8.8.pdf
It seems you need to type in 'Choose from List field' as a 'Value' when setting up the prompt.
You might also want to post this question to the SAP Business One Application space.
-Abhilash