To make leveraged forex trades in IB you need to go to account management and change your account to margin status. It will take upto 24 hours for approval. Once given the permission, any forex trade will dilute your position by 1/33 in the capital i.e. $10K Forex position will approx. use 10K/33 i.e. $330 odd from your account. Rest of the funds will still be available to take other positions in stocks, futures etc.
33 because IB gives 1:33 margin on Forex. This means for roughly $300 you can control $10,000 worth of position in the Forex markets.