The library is not being loaded automatically by LabWindows/CVI. Verify this by opening up the Customize Library Menu: Right-Click the Libraries folder >> Customize Library Menu:
Make sure the checkbox next to your library is checked. If this is a third party library you will need to click on the Add button and add the .fp file of the library:
After this process, click OK and restart LabWindows/CVI for the changes to take effect.