Solution
When using the Modbus API from the LabVIEW Real-Time Module, the Modbus palette will only appear in VIs that are under a Real-Time target in your LabVIEW project. Try simulating a Real-Time target and creating a VI there to see if the palette appears.
You can then move the VI back under the My Computer target.
Figure 1. Data Communications palette under 'My Computer' target 
Figure 2. Data Communications palette under Real-Time target