Up the top left hand of the exchange page (www.mycryptowallet.com.au/exchange) you will see a drop down menu - AUD/BTC, AUD/ETH, etc. Please make sure to click the right crypto before placing your buy/sell. You will also see BTC/LTC, BTC/ETH, etc.
Example below - (this is what you should see)
Now you're ready to buy/sell crypto at whatever price you want! The buy and the sell section work exactly the same as each other. The first input box is the quantity you're wanting to offer eg. 0.1, 0.01, 1 BTC (as you can see in the example below - I am wanting to buy/sell 1 BTC). The second input box is where you set the price eg. $300, $3000 AUD (in the example below - I have left the price at what BTC exactly costs in that moment). The third and last input box automatically fill when you put in your order (total AUD & fee).
Example below -