Enter Redemption Code for Lion on alternate Mac

Hi all!

Have a second hand 2011 iMac which came to me wiped. When I opened it in recovery mode to reinstall Mac OS X Lion, it says an error has occurred please try again later.

After some research I discovered this is because I hadn't purchased and activated my own copy of OS X Lion. Once I do that I can then have the iMac reinstall.

Purchased it and had the access code emailed to me. But as I try to login into an alternate Mac to enter the redemption code I can't find anywhere to redeem it?

At my wits end, would love some assistance if anyone has any tips.