*Sigh*
Don't listen to pianobeast, he just enjoys bashing sibelius.
Anyways... The solution is quite simple. (If you are doing this on one barline).
To make the bass line, look over to your keypad. On the very bottom there should be five tabs labeled 1, 2, 3, 4 and ALL respectively. Click on 2 and enter the bass line.
Then, to create a melody line you can go back and click the 1 and you can enter it directly over the 2nd voice.
If you want a 3rd or a 4th voice, it's the same thing.