We are currently in the process of switching our client over to be compatible with the 64-bit Steam client. In the meantime, please follow this guide to run Steam in 32-bit.
https://hake.me/forums/threads/temporary-workaround-to-run-32-bit-steam-on-windows-x64.34983/
Update 1
The client has been updated to work with Steam's new 64-bit client. The above temporary fix is no longer necessary.
Please download a new client from our website.