In order to enable MEC Single Sidebar, you must go to WordPress Dashboard > Appearance > Widgets.
After that, under the MEC Single Sidebar menu, click on the + sign and search for the term “MEC,” then, select MEC Single Sidebar Items to add the items to the MEC Single Sidebar, finally click the update button in the top right corner.
When the Widgets page is in classic mode, select MEC Single Sidebar Items from the menu on the left, then click on MEC Single Sidebar, and finally click on the Add Widget button.
Now you can manage the options in MEC Settings -> Single Event -> Sidebar Page as described in the MEC Single Sidebar Items menu.
On the Sidebar options list, enable the widgets that you want to be displayed in the single event sidebar.
Note that some widgets, such as Speakers and Map, must also be enabled via the Modules menu to be displayed in the event page sidebar. Otherwise, they will not be shown.