Open the 'System Preferences' application, which should be in the 'Applications' folder on your MacBook.
Click on the 'Sound' icon, which will be in the row of 'Hardware' items within the System Preferences window.
You should see two devices listed there, the internal speakers of the MacBook and the Valab USB device. Click on the Valab entry and the sound should be directed to it rather than the internal speakers.
Sometimes the MacBook will 'lose' the selection you've made previously and you'll have to go back into System Preferences and reset the Sound output to the USB device.