The most common cause of a 403 error is an incorrect token or secret. Verify that you are using the correct values by navigating to the App Settings page in the Dashboard.
If you are using a legacy SDK, released prior to July, 2011, you may notice that your token has a colon (":") in it. You must update the token and secret when integrating with Upsight Marketing's new API and features. The token and secret is available as described above.