Hi fizzyfizz
Ok you can only assign reverb on the aux sends
1. Goto mixer panel
2. Click on return channels
3. Load reverb into an FX slot
4. Turn up the return send for the desired amount
5. Turn and adjust the aux channel on the instrument you want to effect
6. Adjust return if needed
Hope this helps
http://youtu.be/JTif_bYrgq8