You can purchase Platinum Bars by clicking on the “Buy Platinum” button at the bottom of the main menu in Block N Load and then click on the “Buy” button for the size of package you wish to purchase.
When you click on the “Buy” button the Steam Overlay should open. This can take a little while to open up. You will then be able to confirm the purchase of Platinum using the funds available in your Steam Wallet.
You should only need to confirm your purchase once per item you are buying. If you confirm your purchase through Steam multiple times you may end up buying multiple copies of the same package.
If the Steam Overlay does not open when you attempt to purchase Platinum it may be that you have something interfering with the Steam Overlay.
Valve has provided an article containing information on how to enable the Steam Overlay, programs which are known to interfere with it, and how to resolve issues with the overlay in the Steam support knowledge base here.