Generally, this problem is caused when the code is called before the script is loaded. This usually happens when you have the code run as it's encountered rather than after the page load has finished (i.e., in a window.onload event listener). It can also happen if you fire the code after the document is fully loaded but the Feeds API library is being loaded dynamically, etc.
Beyond these general tips, it's almost impossible to nail down what's going on without seeing your page in action. So as I said in my prior post, a link to the page, working or not, would be best.
jg