I thought I'd leave a comment regarding the trail from Rio Frio. Like Tlogan wrote, nobody really seems to know how to get to the top, also from the Rio Frio side. They'll tell you, but there are many trails in that forest, allowed plenty doubtfull moments on which way to go. However, the main trail from Rio Frio starts at Av. Vicente Guerrero. It winds its way up Tlaloc, you could even go up this trail with a 4WD to about halfway. Once you see the top you can leave that main trail and take any of the smaller trails or trailblaze yourself to the top, which I reached after 4,5 hours.