To work with Mitsubishi PLCs supporting the MELSEC protocol (the generation which is programmed in GX Works3 eng. tool), it must be first configured the network configuration and explicitly indicated that there will be a host (in our case WebHMI) in the network that will poll the PLC with this protocol. This is done in the visual design environment of GX Works3. You need to go to the Parameter / FX5U CPU / Module parameter / Ethernet port and 'drag' the required communication modules from the 'Ethernet Device' window:

In addition to Melsec, you need to add SLMP protocol handlers, because Melsec uses SLMP services, it is also necessary to remember the selected IP address and Port No. for exchange.

You also need to check that the Melsec exchange is allowed in the Application Settings:

In connection properties in WebHMI, you must specify the IP address and port from the above properties in the controller project:

Please test first the PLC you're going to use with WebHMI, e.g. with virtual machine demo image, 
some firmware vestions of the  Q/L series have been reported as not supported with WebHMI protocol 
implementation. The driver supports ONLY 3E/4E frame types. 

There 2 things you have to adjust in your Melsec PLC programming tool:

1. State that you are connecting via HUB (not direct Ethernet connection)
2. Reserve more Melsoft connections in the PLC's configuration

Please refer more to the remote service access article.