If you are coming from an external site, to pass the ID of the record you have to format the url as follows:
File Name: SmartStuff, GUID field: SS:GUID
Url formatted as:
http://[your server's address/domain]/[Your Procedure Name]/?change_btn=change&SS__GUID=[actual record GUID]
* note the double underscore between the file prefix and id field name.
Thanks Bruce!