Is it a fill in form (if so, then you should get an i-bar as you move over the fields)? Are you filling it out in Acrobat or the MAC preview? The latter has problems with PDF forms and should not generally be used for this need.
If there are no form fields (not fillable), then you will either have to add form fields yourself (maybe the MAC version has the recognize fields feature of the PC version -- assuming the form is actually text and not just a picture). The other option is to use the Typewriter tool.