In order to find your Groupon voucher you will have to go directly to the Groupon website. First you will sign in and then click the My Stuff button in top right corner of their site.
Locate the deal under My Groupons and then click View Voucher
You will then see a voucher code beginning with the letters GP and you can copy that to code use in the voucher/coupon box at checkout at Collage.com.
If your voucher has been marked redeemed you will have to reach out to Groupon support and they can provide that information to you.
For an in-depth tutorial on how to redeem your voucher code, click here.