Legacy Store Customers are unable to view the Yahoo Store Control Panel link on their Yahoo Small Business Control Panel.
We are aware of this issue and working to resolve it as quickly as possible.
As a workaround you can access Yahoo Store Control Panel directly by accessing:
https://us-dc1-edit.store.yahoo.com/RT/MGR.<STORE_ID>
Note: Replace <STORE_ID> in the URL with your Yahoo Store ID.