Most likely you need to modify your labview ini file to let it know that LPT1 (or whichever) will be controled through the serial port control. See the following link for a knowledgebase answer to this question:
http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/a5abd9d76665217b86256333005ac9a5?OpenDocument
Jared