From the link zenox98 gave:
If you've made a purchase directly from Scirra (rather than from Steam), you will be sent a redeem key by email. This is a special code you can enter to the Scirra Store and get your license.
Once you have redeemed your purchase from the Scirra Store, you can download your Construct 2 license, which is a file called c2license.txt.
If you lose your license file, such as if you format your computer and forget to back it up, you can re-download your license here: