You have example code for this already. Open the example finder and find Write to 1 Dig Line(8255). There is a function on your palette called Write to Digital Line that the example uses. If you want to use a lower level function such as DIO Port Write (and I would recomend), you are correct that you have to send a pattern but what you have missed is the function of the mask input. The mask input enables only those lines that you want to write to. If you want to only enable line 3, then the mask pattern would be 0x08. Your data pattern can be something like 0xFF and only line 3 will go high.