You can add new activities from the top menu Add > Activity. By default calendar entry is selected as the activity group so remember to change it accordingly before saving the activity as you cannot change the group afterwards. Chosen group affects where the activity is shown and what fields and features it has.
Give the activity a descriptive name and all other details. If the activity does not acquire any actions you can mark it as completed by selecting Complete and close otherwise select Add.
You can also add activities in several other views in Severa. For example project's Collaboration tab is a great place to add tasks for the specific project.