Make sure you have saved the task (or any other CRM record) at least once before you attempt to add an attachment onto it. Since the record won't exist in the database prior to the initial save, CRM will hide away the paperclip button for adding an attachment,
as the required parent object would not be available for the "add attachment" dialog to complete successfully.
Jukka Niiranen - My blog: Surviving CRM -
Follow @jukkan on Twitter