Baught the corona bundle on humble bundle, how do I use free month premium subscription?

Do you already have a sub, or once subbed?
Cause if so, no coupon for you!

If you click on the top right of the page, you should have a 'Coupons' option in the dropdown. Check if it's there, if it is then just purchase the bundle and the coupon should be automatically used

https://www.humblebundle.com/user/subscriptions/humble_monthly/billing
check here if you ever subbed before
otherwise, contact support