Adobe Reader?

Are you using Adobe Reader or Apple's Preview?

Assuming you are using Adobe Reader, all you need to do is select the "type" tool (It looks like a vertical I-bar). Next select any areas that allow inputed text.

If that doesn't work it could be a compatibility problem with the way the PDF file was originally saved.

 

If this is the case and it dosen't work, check your Adobe version, this is a somewhat new feature to Adobe Reader.

 

And if you are using Reader, make sure you print the form after filling it out, because you CANT save the info you entered