I've just run across this perplexing problem. On a Windows machine, when tabbing into a pop-up menu field, the pop-up shows with all its options. But, on a OSX machine, the same tabbing will not show the pop-up at all. In fact there is no visual representation that the field is active so pure keyboard entry cannot be used for that field. Is this a bug or am I missing something?

You'll have to either use a drop-down list or type-ahead, or take that field out of the tab order to avoid confusion.