Elder Murphy told me that DearElder is not as effective in Mexico as it was in Provo. He told me of a new alternative: http://www.mtcmexicocarepackage.com/home.html This website is run by members of the church in Mexico. You can send him packages or a letter. (Update: Since Michael is no longer in the MTC, this will not work. See his new address here.)
To send him a letter you must make a PDF. Here is a tutorial.
Then you send the PDF to this email firstname.lastname@example.org
and write his name in the subject line