As much as I hate to necropost, I believe this is for a good cause. I had this problem too recently, and the only result I could find when googling the error was this thead. I managed to solve this problem, so I'm going to post the solution here so that anyone else who googles the error will come across this thread (and thus the solution).

1: Install the regular version of Spiral Knights (It doesn't really matter where you install it to).
2: Open up the folder you installed the regular version of Spiral Knights to
3: Browse to (I am assuming you are on a 64-bit machine running Windows, with Steam installed to the C:\ drive)
C:\Program Files (x86)\Steam\steamapps\common\spiral knights

4: Copy "getdown-pro.jar" from the folder with the regular version of Spiral Knights to the folder with the Steam version. If it asks you to overwrite, confirm it. If it does not ask you to overwrite anything, you're probably in the wrong folder(s).