By now you've probably already figured it out, but in case others have the some issue (I know I had):
SPOILERS:
In order to access Mateo's route, you need to get onto Sergio's.
Sergio's route-Ignore the Door, Peek Through The Peephole.
When you do this, Mateo will soon come as a repo-man. In order to branch from Sergio's route to Mateo's, you must be kind to Mateo and agree with him. Specifically, you must choose "Greet" for option 6, "It's home" for option 7, "Introduce" for option 8, "He's Scary" for option 9, and "Leave" for option 13.
Do keep in mind that the developers recommend waiting until you've completed every route other than Anise's before playing his route, as playing his route too early will spoil some of the mystery in other routes. His route ties a lot of other routes together, so it's really meant to wrap things up before the finale.