Are you not able to see the + button to add activities? Then you have probably enabled 'view only' when logging in. On the Tiimo app, you're able to login in two different ways:
1st way is with 'view only' toggled off
This is either for those who plan and use Tiimo themselves, or for those who plan on behalf of another user.
2nd way is with 'view only' toggled on
This is for those who don't plan themselves, or should not be able to create or edit activities, and instead only view the Tiimo calendar. This feature is great for many of our joint-users, like those with younger kids who are not yet involved in planning. Plus it makes the user experience more streamlined, without any potential stress or indecision induced by planning features popping up.
If you enabled 'view only' by mistake, just log out, toggle it off and log in again.