Error message:
"You cannot create any more free accounts from this computer"
1. Installed game from steam
2. Marked the "Use Steam Account" and pressed Logon
3. Error appears.
A different steam account made no difference. A registration @ http://www.spiralknights.com/ made no difference.
If anyone else has already played the game on the computer, it may have already hit the 3 account limit for a machine.
Details on account limits are intentionally sketchy, so your best option at the moment is to submit a support ticket.
https://www.spiralknights.com/support/#mail
In the mean time, you should be able to play using the web account user/pass, and merge it with your steam login later on. The only difference is you can't collect steam achievements, so if your only goal is to play, you don't need the steam login.