Your IB account is a multi-currency account. Suppose your account is in EUR and you have placed EUR cash in your account to trade European stock. Then you decide that you want to buy some US stock, which is obviously being traded in USD. IB allows you to open your first US trade and your account will now show both a EUR cash position and a USD cash position. Your USD cash position is negative because you have used USD to buy the US stock. In fact: you are borrowing this USD cash from IB. You will see both cash positions in TWS listed separately. Your EUR cash position is not affected by this USD trade. If you don't want this negative USD cash position you have to place an FX order to buy USD, using some of your EUR.
If you don't buy a US stock but a US futures contract the story is similar but different. The commission for the contract is charged in USD, but this is only a small amount (e.g. 1~2 USD). However, in order to keep this position you need an overnight maintenance margin, which is charged in USD. IB puts up this maintenance margin for you and charges it back to you. This, however, does not show up as a negative USD cash position! You won't see this displayed margin requirement in TWS. You will see this in your monthly statement, as there it is mentioned that you had a negative USD position and that IB charged your debit interest over this USD position. Your monthly statement will display an overview of each of the currencies that you hold in your account. Plus a combined overview in the so-called BASE currency (probably EUR in your case).