At first I imagined that this was the problem, but Xamarin wont let me to add Mono Runtime at that screen so I thought that this was not the problem then.
Today I re-opened Xamarin and it ask me to update itself. I updated Xamarin to the last version, unistalled Mono, re-installed it again and it appeared then in that list of .Net runtimes.
Now it is running flawless. Steffeler, thank you so much. I really, really appreciate your answer.
- This reply was modified 3 years ago by Samuel Otero.