What version of Poser are you using?

Try changing your element display style. Select the hair object and then go to your Document Display Style pallet. Click the arrow beside it and select Element Display Style. Then select a display style that lets you see the object better. Hair usually has transparency maps and they don't always show very well in full textured preview. So going to smooth shaded or wire frame can help. 

If there hair is made for the figure you're using, then it should adjust on its own with the head shapes that you use. 

 

 There is a series of tutorials here: 

http://www.renderosity.com/mod/tutorial_premium/index.php?page=info&tutorial=10

but they are for Poser 10/2014. Most of the same techniques will still apply for rendering if you're using P7 or higher. There are also dozens of tutorials on lighting and rendering here in the tutorials section.

 

ETA: Gave the wrong link before. Sorry about that. The link above now goes to the Materials tutorial. 

 

~Shane