Simply press the + key for Time; However, if the controller determines that the time is insufficient for opening/closing the light shutter, it will add time while giving you the correct exposure.
Adding time to what the controller determined to be the correct exposure WILL make the print darker as you would expect.
This is more difficult to explain than to actually do. Get to know it by using it - unfortunately I no longer have a manual but there is a thread on APUG where a manual is being offered. DURST-PRO USA also has manuals online. Durst (in Italy) has a manual available as well.
PS - Where are you located? If you're in the USA, I can walk you through by phone.