Ben - Your new hard drive does not have the needed software on it yet.
Do you still have the USB thumb drive? Or if you have the external optical drive and the recovery CD/DVD that came with your system?
If not is your old HD still working? If it is you could try using a universal drive adapter to connect it via your USB port.
Using any one of these you should be able to install a fresh copy of your OS onto your new drive.
If you don't have any means to recover things, you still have an out. See if you can prep up a USB thumb drive off of a friends mac. The one issue here you'll face is your system can only support 10.7.5 (Lion) or older (Snow Leopard being the better OS). If you can't find someone with the needed OS your last hope is to call Apple and order the recovery disk/USB. It will cost you around 20 dollars (you maybe asked for your systems S/N). If they send you a disk you'll then need to locate an Apple external optical drive if you don't have one. Once you have the needed means to install the OS the rest is easy ;-}
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