A single registered development user can have multiple applications. Each developer application has a unique consumer key. A Consumer Key is required to authenticate your developer account with a GoTo account.
To find the Consumer Key of your app go to My Apps on the Developer Center. You will see a list of all your apps. Click on the app you need the key for and select the Keys tab. It displays your app's Consumer Key, Consumer Secret, and Callback URL. At this time the Consumer Secret is not used in the API.
To use the key to obtain an access token, see How to Get an Access Token.