In order to allow a document to be digitally signed in Reader, it requires so-called Extended Rights. Normally, Reader does now allow digital signatures to be applied; this is an implementation limitation Adobe imposed on Reader.
I think you can use Acrobat Pro to assign these Extended Rights (but I am not completely sure). If not, you will need Reader Extension Server, a product of the LiveCycle line (hint, get your big purse ready for that).
Hope this can help.
Max Wyss.