If you uploaded the file to be signed on AMO (see this doc if you haven’t), you should have received an email with instructions on how to download the signed file. As for the hash, most operating systems have command-line tools like shasum that can generate the hash from the file.
Yes, I read the documentation and no, I don’t have the link to download the signed file in the mai received after submition to AMO (but well the link to the detail page).
I will try to find the equivalent of shasum statement on Windows 7, but first of all, I have to have my XPI file.