HQ Credits is the prepaid balance your account uses to pay for usage-based features, starting with SMS.
Two things have changed:
Note: what used to be called your SMS balance is now HQ Credits. It's the same balance — your current amount carried over automatically. The new name reflects that credits will cover more than SMS over time.
Go to Settings. Your balance is shown in the button at the top right, and the HQ Credits tab holds your full history.

You need the Settings permission to buy credits or view the history.