Press the FX button.
Go the FX3 tab.
Press the Fn Key, press “Library,” select the FX you want (eg a gated verb,) and press “Recall” to load it.
Press the Back Panel button.
Select insert.
Select the relevant channel/mix you want to insert the FX across.
Press Apply.
You’ll get a Dry/Wet control.
If you want to insert the reverb across the whole mix you’d do the above but instead of selecting the snare channel you’d select the relevant mix. If you want to route a reverb return at a particular mix (more likely than an insert,) you’d go to the top layer, press the relevant mix button on the RHS of the console so your fader were mixing to the relevant monitor mix, then you’d raise the FX return fader from the relevant FX engine. pp50-52 of the v1.7 manual explain this.
HTH