Thanks for playing and for your feedback!
Did you try clicking on one of the active keys? When clicking an active key, if your constants aren't full, then it switches that active key to a constant, or you can disable all together.
You probably figured this out, but active keys are the ones that fall, and count towards your attack/armor when submitting a word, where constants can be used at any time but don't add to those points; however, both add towards your word and it's multiplier.
You can have a max of 5 active, and 5 constants, and any of the keys can exist as an active or constant.