The official resource to create dll's or using external code in LabVIEW is the document "Using External Code in LabVIEW". This document should come with LabVIEW. You can download it from ni.com at:
Using External Code in LabVIEW
http://digital.ni.com/manuals.nsf/web_productcurrent/4F1447F7CD83D6D88625690D00637CED?OpenDocument
Best regards;
Enrique
www.vartortech.com