Need to create an IDX link? See this guide: Saved Search IDX Links
In order to refresh the IDX link list in the WordPress plugin, we must clear the cache.
Go to Flexmls IDX > Settings > Clear Cache

In versions before 3.13...
Go to FlexmlsIDX > Settings > API Settings > Clear Cache

Once the cache has been cleared you should be able to select your new IDX link(s)…

- Selecting (Use Saved Default) refers to the IDX link you have set under Flexmls IDX > Settings > Behavior
The cache in the plugin will automatically clear every hour. Fore best results do it manually.