Step 1: Open the LabVIEW Help.
Step 2: Click on the Index tab.
Step 3: Enter dll in the search box.
Step 4: Click on the appropriate index entry. Like the one that says "DLLs calling from LabVIEW".
Step 5: Read.
Alternative:
Step 1: Open the LabVIEW Help.
Step 2: Click on the Contents tab.
Step 3: Expand to the Fundamentals->Calling Code Written in Text-Based Programming Languages.