Datepicker Calendars won't pop-up
The Extension has some Modules as well as some Views that use the datepicker calendars for letting the customers choose the Participation Date.
When such fields get the focus by clicking on them, the datepicker calendar should pop up. If the calendars are not being displayed when clicking on those fields then there is a Javascript conflict on your website with other modules/plugins that is breaking this function.
From the Configuration page of VikEvents it is possible to select the Calendar Type to be used for the front-end. There are two types of calendar, the Joomla Calendar and the jQuery UI Calendar. The default calendar type is the jQuery UI and that's the one you should be using to benefit of all the functions of this calendar. In fact, the Joomla calendar cannot be configured properly such as setting a Minimum Date, a Maximum Date or disable some days of the year.
In order to fix the Javascript conflict and have the jQuery UI Calendar working normally, visit the section "How to fix Javascript Conflicts" of the Documentations page.