I am told that I have positive purchasing power, but negative cash in a registered account. What do I do? 

The purchasing power of registered accounts combines the cash in your Canadian and U.S. dollar accounts. However, when you place a purchase, you must ensure that you pay for that transaction in the corresponding currency on or before the settlement day. 

For example: you have $5,000 CAD in your Canadian TFSA and $0 USD in your American TFSA. You buy $2,000 USD worth of ABC stock in your American TFSA. Since the share settlement period is 2 days, you have 2 days to convert part of your Canadian dollars into US dollars to pay for your purchase.