Pull down the "Advance" menu and select
"Extend Features in Adobe Reader"
A dialog box will come up that says "This document already
has enabled usage rights in Adobe Reader. To remove Reader Enablement,
use File > Save a Copy".
Follow these instructions, they will allow you to create a copy of the
document that is not restricted and has no extended features in Adobe Reader.
Rename the file when you save it. Close the current file and open
the new copy.
If you still can't edit the fields,
pull down the "Forms" menu and select
"Add or Edit Fields"
A dialog box will come up asking if you want Acrobat to detect form fields
for you. Click "Yes"
Make any changes to the form that you would like and save the file.