Navigate to FlexmlsIDX > FlexmlsIDX > Account under the menu in WordPress.
Enter in the key/secret provide to you in the getting started email and hit 'Save Credentials'
If done correctly you should see a green 'connected' label:
Google Maps Key (optional, yet recommended):
In order for maps to display on your website, you must include a Google Maps API Key:
https://developers.google.com/maps/documentation/javascript/get-api-key#console
It is best to follow Google's documentation for the Map API key.
After you have setup your key navigate to FlexmlsIDX > Settings > Google Maps:
Default Map Height: Enter a height value in px. (i.e. enter 350px)
Google Maps Javascript: If checked, the Google Maps JavaScript will not be loaded by the plugin. Use this if your theme or other plugins already load the Google Maps Script and your API Key.