There is a known issue while activating that defaults the "Deactivate API Key" setting to a selected status, making it impossible to activate the plugin. To fix this, simply navigate to the "API Key Deactivation tab, and ensure this box is NOT selected. Click Save, and you will then be able to activate the Key on the API Key Activation tab. See below for reference.
After ensuring this setting is disabled, click Save. Then, navigate back to the API Key Activation, and activate your API using your key (which is located in the API Keys section of your account area). That should cover it! MapifyPro is now properly activated and your maps will display as expected.