For those using a Mac and unable to login directly with the Steam account if there is no checkbox I've managed to find the solution to the problem. It turns out the newest version of Java, SE 6, for Mac is unable to recognize the Steam Login checkbox for Spiral Knights. Here's a few screenshots showing when I changed them.
Running Java SE 6: http://i316.photobucket.com/albums/mm349/borntobuild6/Picture2-1.png
Running J2SE 5.0 (64-bit): http://i316.photobucket.com/albums/mm349/borntobuild6/Picture1-1.png