The rx_seriallpbken port does not appear when you select the serial loopback option on an ALTGX Megawizard® Plugin Manager for Cyclone® IV GX devices because dynamic control of serial loopback is not natively supported in Cyclone IV GX devices.
To dynamically control serial loopback in Cyclone IV GX devices, you can generate a MIF file for each ALTGX Megawizard Plugin Manager configuration and use MIF based dynamic reconfiguration to switch between the two modes.
MIF based dynamic reconfiguration for Cyclone IV GX devices is explained in the following application note.