A StaxelKey is a game code that is needed to turn your staxel account into a premium account.
it can be bought through http://blog.playstaxel.com/store
If you bought Staxel through HumbleBundle.com the staxel key is on the redeem page above the link to the installer, you can go back to the redeem page through the humblebundle.com email or through your humblebundle.com account. If you can't find the link you can try https://www.humblebundle.com/resender
The StaxelKey is entered during registration of the game account in the launcher. (or redeemable after logging in for alpha accounts)