What version of FileMaker are you using?
Pop up menus in FileMaker 12 automatically show an arrow on any but the "classic" layout themes. In FileMaker 11, you might use a drop down list instead of a pop up menu and then select the arrow option for it.
Thanks, Phil. I do have 12, but was using the classic theme. Ir was easy enough to grab a theme for just the dropdown and get the arrow. I think it' s strange, though, that with all that CSS control over objects, you still have to do all sorts of workarounds like that to get it to look right (I even had to narrow the field by a couple of pixels because FM only has the arrow top-aligned, not middle-aligned...)
Themes are new with FileMaker 12. It would not surprise me if the next version introduced additional functionality when it comes to working with them.