To set RX as the Sample Editor for Logic Pro X:
Select Logic Pro menu -> Preferences, and then click on the 'Advanced' tab.
Enable the 'Audio' check box.
Click on the ‘Audio’ tab, and then select the ‘Audio File Editor’ tab.
Click in the ‘External Sample Editor’ path, and navigate to your applications folder. Select RX and click ‘Choose’.
To edit audio from Logic Pro X in RX:
Select the audio file.
Click Edit/Open in iZotope RX (the keyboard shortcut is Shift + W)
RX will automatically load.
Perform your audio edit, then click File/Overwrite Original File to save the edit.
Close RX, and navigate back to Logic Pro. Logic will automatically update the audio file to reflect your edit.