Hey Inês ,
I understood your problem.
Please follow the below steps :
1)Create Selected_Month and Selected_Year local variable of type integer.
2)In MonthPickerOnSelect drag and drop assign and assign the year.But for Selected_Month use
If(SelectedMonth.Month=Entities.Month.January,1,
If(SelectedMonth.Month=Entities.Month.February,2,
If(SelectedMonth.Month=Entities.Month.March,3,
If(SelectedMonth.Month=Entities.Month.April,4,
If(SelectedMonth.Month=Entities.Month.May,5,
If(SelectedMonth.Month=Entities.Month.June,6,
If(SelectedMonth.Month=Entities.Month.July,7,
If(SelectedMonth.Month=Entities.Month.August,8,
If(SelectedMonth.Month=Entities.Month.September,9,
If(SelectedMonth.Month=Entities.Month.October,10,
If(SelectedMonth.Month=Entities.Month.November,11,12)))))))))))

It will definitely work!!!
Thanks & Regards,
Sudip Pal