I understand that you are having some issues in order to add a Slingbox to your Sling Account.
You must go to accounts.sling.com and log in to your account. Then, select the last option on the left hand side menu and go to "Slingbox Directory". Click on the "Add a Slingbox" button and make sure to enter the Slingbox ID and all of the requested information.
This way, you will be able to add your Slingbox to your Sling Account. If you are not able to get the Slingbox ID, the following links will provide you with all the required information: