Which "Enter"? The Enter key that is part of the QWERTY section of your keyboard or the Enter key that is part of the number keypad?
Well, I have a MBP, the one that is part of the QWERTY keyboard
That key can't be used to perform the Find in IWP as far as I know. In regular FileMaker, it can't perform the find unless you add a script trigger and script triggers can't be tripped by direct user action in IWP.
The numeric keypad Enter, on the other hand can perform the find in regular FileMaker so it may work in IWP.
To make sure that users can perform the find, I recommend making sure that a button, either one added to the layout or one visible in the tool bar is available for performing the find.