Hi Marcelo,
In order for an Activity to show up on your Activities Calendar you would need to make sure this Activity type has the option checked off for "Show On Calendar?". You can find this out, or modify this value by first going to Customers->Activity, and choose the "NewActivity" screen. From within the NewActivity screen, locate the 'Activity Type' dropdown field and click on the ellipsis button on the right hand side of the dropdown (the 3-dots), this will load a pop-up screen labelled "New / Edit Activity Types".
When this new pop-up screen is loaded, you can select the Activity Type you want to verify from the grid at the bottom, this will populate the fields in the top area with this specific Activity Type. Notice the column in the grid labelled "Show on Calendar", this is what decides what would show up in your Activity Calendar and what wouldn't. The only surprising thing for me here, is that the default option for the "To-Do" activity type is yes, to show in Calendar.
Can you please first go to this screen described above and make sure that your 'To-Do' Activity Type has the option "Show in Calendar" set to Yes?
-Ben