If you are getting an error that says "This application has been authorized" when attempting to log into any of the SketchUp websites (forums, 3DWarehouse, Extension Warehouse), most likely the solution is to change your internet browser's security settings to allow cookies from 3rd party websites.
On internet Explorer we recommend a "Medium" level security policy. See the section on "Change your Security zone settings":
On Chrome we recommend putting your Content settings for cookies to "Allow local data to be set". See the section on "Allow cookies by default":
On Safari we recommend putting your Privacy settings for cookies and website data to "Always Allow". See this article about managing your security settings: