Go to setup > Playback Engine.
Under the "Current Engine" list do you see the mbox?
If so select it and click OK.
Pro tools will want to restart. Let it, then go to Setup > IO.
On the input tab and the output tab, select default.
If you didn't see the mbox in the list, go out of Pro tools and go to Utilities > Audio/Midi Setup.
Find the Pro tools aggregate device and click the check box to add your Mbox to this device.
Then go back to Pro Tools Setup > Playback Engine and select the aggregate device.
Reset IO to default as above.