For information on how to access your listing dashboard, see this article.

Go to the Listing Dashboard. You will see your MLS information at the top, beneath the property address. The MLS you have chosen is loaded from your previous listing, but you can always update by using the MLS settings button:

After you click on "MLS Settings", enter in the MLS number in the text box provided. Then, select your MLS provided from the drop down: 

Once you have chosen your MLS Provider and entered in the MLS Number, don't forget to click the Update  button at the bottom of the screen:

Need help getting MLS Sized Images to upload to your MLS Listing? Check out this article here.