Strange - does it even make sense there should be a sample rate selection, the program is working with MIDI & timing data, not audio... (!)
Edit: Ok, it does say (can't copy/paste from AppStore, grr...) "Select Project Sample Rate to match the current project setting", for some reason.
In any case, whatever the sample rate you are working at in your project, that's the option you should choose in that plugin, presumably for some timing reasons or something.
If the timing is out at a particular sample rate, maybe contact the developer and give them a bug report...