Hello,
As far as my knowledge goes, I do not believe that there currently is a way to link a help file to a LabVIEW .dll. However, it is possible to link a help file to a LabVIEW VI. This can be done by going to File >> VI Properties in LabVIEW to open the VI Properties page. Then in the VI properties page select the Documentation category. Here you can enter a VI description and a link to a help file. Then, in TestStand, when you go to Specify Module for the LabVIEW adapter both the VI description and the link to the help file will be available. I hope that this information is helpful for you.
Regards,
Kevin L.
Applications Engineer
National Instruments