Not a clue what your voucher code is for, but I can tell you where there's a place to enter voucher codes. I hope that's helpful.
If you click on the Shop button you get the Shop screen. Underneath the row of buttons across the top is the option "Redeem voucher" in little black letters. Click that.
It takes you to a page where you can enter a voucher code and click Redeem. The text on that page claims: "Only a select few in Days of Evil can lay claim to runes of power. Are you one of them? Then make your mark in the field below. If the symbols successfully unfold their magic, then you'll soon be swimming in wealth."
Let me know what it feels like to be swimming in wealth.
NOTE: I guess you have to be level 5 to see the Shop button, but I'm pretty sure you are. I'm just adding this in case any lower levels stumble across it and want to know why they can't see the Shop button yet.