Start without the barlow, until you've perfected the technique.. Webcam astrophotography with a non-tracked dob is very difficult, and frustrating.
Once you've got better at lining it up without the barlow, then chuck the barlow in and watch the frustration increase
It all comes down to lining up your finderscope with your CCD image.. so without the barlow, get Jupiter in the FOV of your CCD, nudge/push/pull until it's in the middle, and then align your finderscope so that Jupiter is right in the middle of the crosshairs...
Then, once it has drifted out of the FOV, you can nudge the scope to put Joop on the "other" side, allowing it to drift through the FOV of the CCD.
Remember to orient the CCD so that it drifts across the horizontal direction (640px) as opposed to moving up or down, which only gives you 480px in which to capture.
With Jupiter so low, and disappearing so quick after sunset, it's going to be a very difficult task for you. Maybe you should keep practising on the moon until Saturn gets a bit higher
anyway, everything I said above is in my article: Astrophotography with a Dob
Hope it helps!