This is not a preset letter, so the best thing to do would be to compose your own, but I have copied my note below, which you may borrow if you are short of time.

You may want to research this topic before deciding to act, I would like to suggest these links:

http://persian2english.com/?cat=23

http://www.reuters.com/article/idUSTRE61B3JY20100212

The link for contacting the International Secretariat of Amnesty International is next, please note that an address where you can send a pen and paper letter is also available should you prefer that:

http://www.amnesty.org/en/contact

The text I used:

Thank you for your recent statements with regards to the status of humans rights and rights violations in Iran.  I would like to encourage your to continue and even to increase your activities with regards to this, even though with no official presence in Iran this must be a daunting task.  I hope that you accept or solicit for the names of persons who have been detained by the Iranian government for political speech, and that you will sponsor letter writing campaigns on their behalf.
Thank you again for all that you do.
Best regards,
Mary A. Dearing.