You could ask taxi drivers at Penang Ferry Taxi Stand or Butterworth Taxi Stand. I will update the article shortly to add in the taxi stands location.
It will be quite expensive to take a taxi direct to Lumut.
You could take a bus from Penang island or from Butterworth. To take a bus from Butterworth, you need to take Penang Ferry to Butterworth, the bus station is just near by to Butterworth Ferry Terminal.
Running a check on taxis from Penang to Lumut - not many, of course there are, but you may need to find them. Even if you find one, it will probably be quite expensive. Maybe a better deal is to take a bus from Butterworth (across Penang Island) to Lumut.