Cybertaz is right - if your printer prints 2-sided, then you need to specify
this format in the printer properties. In the Print dialog box, click the
Properties button and look for a "Print on Both Sides" or similar option.
If your printer does not print double-sided, however, you can tell Word to
print all Odd pages, then flip the pages over and have it print all Even
pages. In the Print dialog box, change Print: <All pages in range> to Print:
<Odd pages> / <Even pages>. Depending on how your printer spits out the
pages, the stack of Odd pages may need to be re-ordered so that p. 2 prints
on the back of p. 1, etc. (my printer at home spits them out face-up, so all
I need to do is flip the stack over.)