The most common support request for Statistics for WordPress is from people struggling to find the details for their site.
To this end, I have written a very small WordPress plugin which displays the API key and URL used by the WordPress.com stats plugin.
Download and install the stats helper plugin (link below) and you will find a new item in the ‘Tools’ menu:
Click this and your details will be displayed. These should be entered into the iPhone app exactly as you see them on-screen. Now the app is ready to go!