There are a few reasons why you may not be able to transfer your AUD into USD.
You have less than $50AUD in there.
As there is a minimum FX transfer of $50AUD, you will need that or more to transfer into USD.
Macquarie has not completed your account
Even if you have been approved through the electronic verification, Macquarie Bank will undertake their own checks to finalise your AUD wallet, which takes 1-2 business days.
You may be stuck in the situation where you have funded your AUD Wallet, but are still waiting on Macquarie's checks to be completed until you can transfer $ from your AUD Wallet (Macquarie CMA) and into your USD trading account.