The dll you are referring to may need to be registered. The following webpage shows how to do this:
http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/4f811a9b23f1d46e862566f700615b7a?OpenDocument
When trying to locate the ActiveX controls that I want to use in LabView one thing I do is open up the registry and search the HKEY_ClASSES_ROOT\TypeLib folder for a key word -- in your case a good key word would be input8. If the search finds a subfolder that contains the search word then look through its sub-sub folders and you should find an entry referencing a path and a filename -- it's usually a dll, ocx, olb, or tlb file. Then, in LabView, drop an automation refnum on the front panel, right mouse click on it, select 'Select ActiveX Class' >> Brows
e... Click the Browse button and navigate to the ActiveX file. Click Okay. You should now have access to the ActiveX object's properties and methods.