API Key not working

Once I figured out (thanks to these forums) that I need to sign up for a wordpress.com blog to get an API key, I did that. I took the key and put it in on my original, hosted blog. But I keep getting an error message saying my key appears invalid. I’ve tried logging out, deactivating the plugin, reactivating it, typing the API Key in manually, copy and paste, but I keep getting the same error code. Any hints?