get the people across the road from the Consulate to do your paperwork..quite cheap and they know what you need..maybe check your address is correct on form
you can do yourself..get a mini van from Mo Chit to Kanchanaburi..and there are buses to border from bus staion in Kanchanaburi..but I don't know about Mexican passport so am guessing it is ok..cost through border is around 960 baht