TradeKing uses an authentication mechanism that is incompatible with our aggregation service. We are researching alternatives but we don't have an ETA on when these accounts can be linked in Personal Capital. Apologies for the inconvenience.
In the meantime, you can add this account as a manual investment account to Personal Capital so it can be included in your Net Worth and other calculations. For instructions on how to add a Manual Investment Holdings account, please see this help article.