I redeemed a discount I received in the GOG newsletter. Once I entered the discount code it said I had 30 days to complete the purchase. I went back in a day later to buy the game and I cant find out how to access the discount. Any suggestions.

I think you have to redeem it again when you decide to purchase, at least that's what I did when I didn't want to commit to purchasing a game right away.

Go to the Redeem a code page and enter it again.
If the code shows up as used, you can probably find a working one in the Newsletter discounts thread, as long as the offer hasn't expired yet.
If the offer has expired, open a support ticket. Supposedly staff can help when discounted items can't be purchased due to technical issues on their end.

This codes are one use only. When entered it's lost, doesn't matter whether you bought anything or not.