Unfortunately your available balance for new orders isn't displayed on the site. We realize that this is very inconvenient and we're working on adding this feature.
In the meantime, you can get an idea of your available balance for a specific currency by looking at the "withheld" amounts on the withdrawal page for that currency.
For example, if you want to buy a cryptocurrency using EUR, you can go to Funding > Withdraw > EUR and look at your "withheld" amount. Your "withheld" amount is usually the amount you have tied up in open orders, so that number subtracted from your balance in EUR will tell you how much EUR you have left for new orders.
If your balance in EUR is less than your verification level's withdrawal limit, then the figure listed as your "maximum withdrawal" amount will be the same as the amount of funds you have left for new orders.
The same goes for other currencies - you can look under Funding > Withdraw > [currency] to see how much is withheld in that currency.