In the web interface, it's in the menu on the left, Setttings/ General, scroll down a bit to Active Event, and change to "current official event", then restart your server. But keep in mind, event dinos will only spawn occasionally to replace normal wild creatures that have died. If you don't want to wait for them to spawn, load into the game, open the in-game console, enter "enablecheats <password>" (putting in the admin password you set ), then "admincheat DestroyWildDinos". That should make all the wild creatures go poof, and as new ones spawn to replace them, a certain percentage will be the "eerie" ones.
By the way, the same settings as the web interface are also in the Nitrado mobile app, you just have to drill into it a bit.