This is just the snip of one of my steps (within POST method). I've tried adding within the GET method, as well, but that gives a null result.
if (context.request.parameters.curstep === '2') {
log.debug('curstep 2');
//createAssistantSteps(data, "12", system_type);
createAssistantFields(data, ['12'], system_type);
ConfigSublist(data, ['12']);
var sublist = assistant.getSublist({ id: 'custpage_configuration_sublist' });
log.debug('assistant.getSublist() 1', sublist);
assistant.currentStep = assistant.getNextStep();
context.response.writePage(assistant);
}