You may bring pseudoephedrine Sudafed in for personal use. From the customs website http://www.sat.gob.mx/aduanas/pasaje...ar_Mexico.aspx "medicamentos de uso personal (en caso de sustancias psicotrópicas debe mostrarse la receta médica)." -> medications for personal use (in the case of psychotropic substances a prescription must be shown).
So just mark "Yes" in the customs form where you declare if you are bringing medication with you and, since pseudoephedrine is classified as psychotropic, be prepared to show it and the prescription upon request.