If you are going to write the LV code yourself, you should read "Using External Code in LabVIEW." You can find it by opening LabVIEW >> help >> search the LabVIEW bookshelf. This is the best source of information concerning calling dlls.
You may also want to look at a LabVIEW 7 shipping example, call dll.vi. You can find it by opening LabVIEW >> help >> find examples >> Communicating with External Applicaitons >> Using External Code>> integrating dlls. It is very helpful.