As the FX send is not PRE/POST switchable, you cannot hear only the wet signal from the FX return without hearing also the dry signal from channel 1 as the FX send is fixed to work as a post-fade send (which means after the channel fader. If the channel fader is down, no audio will be sent to the FX unit).
If you purchased the mixer within the last 30 days I would recommend you return it to the retailer in exchange for a QX1204USB, which features two aux sends per channel. The second goes to the internal FX unit post-fade as per usual, but the first is PRE/POST switchable, which is what you need. You will need to connect your FX unit to the AUX SEND 1 output, set the switch to PRE and close the channel fader, to hear only the wet FX return.