Update:
I just fixed the transistor-bug.
Apparently I forgot to connect the transformer's (PSU) ground to the Arduino's Ground. This caused the transistor's control pin not to get any power AT ALL.
If there's on thing I always forget then it's that.
So I got the control circuit working, though I do have a new bug now (if the power is set to off then the light stays on, but very dim though).
EDIT: Fixed everything, circuit works as it should now.