If you use Google Maps, you can see that you shouldn't get lost from Martinez station. It's basically two straight roads - Alvear, and then turn left onto Libertador until you hit D Repetto. The beauty about the road systems in BsAs is their simplicity of blocks and numbering, very hard to get lost IMHO.
I will forward my cell number to those who may need it, just in case.
One thing I would warn people getting the train from Retiro - Take coin to use the machines, the lines for the ticket office can be vey long on a Sunday with locals heading to Tigre for the day, so factor in a little extra time, just in case.