I can't print double-sided

[Pacopag]

Hi. The printer at my school can print double-sided, and for most people around here, that is their default. But I can't seem to do it. I am using whatever program is the default for pdf's. When I go to 'Print', under the 'Page Setup' tab there is a label titled 'Two-sided' with a drop-box, but the drop-box elements are 'One sided', 'Long edge binding', and 'Short edge binding'.

The printer is a HP LaserJet 4250dtn.

[anubhavrocks]

Select Long edge binding.
Also you can try

 hp-toolbox

[Pacopag]

Alright then. That couldn't have been easier. Thank you so much for your simple, yet effective, answer.