We just published an update of the Statistics Extension that should make it compatible with RapidMiner 7.3. However, several restrictions of the new RapidMiner Security Mechanism are preventing existing functionality to work. While all operators are working as expected, you will not be able to enter the license key in the dialogue. 

Hence for now you need to go over the RapidMiner Settings / Preferences menu, switch to the Licenses tab and past your email and license key there as shown on the right.