Once you've deposited to your account you need to transfer the funds to the respective margin account to start trading. Read about margining.
The transfers section is located in the bottom left-hand side of the platform:
Clicking the arrow next to the relevant wallet will open a dialog box to process an sub account transfer:
The destination depends on the instrument you wish to trade. For example, Bitcoin can only be transferred to margin accounts which use Bitcoin as collateral (XBT:USD Margin and XRP:XBT Margin). These funds can then be used to trade PI_XBTUSD, PV_XRPXBT, FI_XBTUSD and FI_XRPXBT. Once transferred the available margin will increase, allowing you to start trading:
The balances will also update in the transfers section:
In order to request a withdrawal, funds must be transferred from your margin account to your cash account.