Your current BTC balance might be available on your Multi-Signature wallet. You can check all your balances through the accounts page: 

To be able to utilize the amount for buying/selling, you will need to transfer the BTC from your Multi-Signature wallet to your Core/Pro account. You will be able to transfer it through the following link: 

Select "BTC Core/Pro Account Balance" from the "Withdrawal Recipient" dropdown list: