I had the same thing, until I thought to ensure “Publish to the Web” was set on my Google Spreadsheet (under the File menu). Does that take care of it for you?
Hi Allison, I am the plugin author. Sorry the delay. For any reason I am not getting new support notifications in my inbox.
Can please contact me by email? ( info at magn dot com ) I can personally help you to troubleshoot this. As @canuckinmi says, we need to make sure to Publish to the Web, there was a change in the Google Docs API after the plugin was developed that affected this. If this is your case, I may need to update the instructions.