http://www.gmayor.com/SelectFile.htm should provide a few suggestions. The problem as always is that you cannot force users to run the macros required.
One possibility that may work with the Mac version (I don't work with Macs) is to use the calculate on exit property of the form field in conjunction with a REF field, but it will only work with the REF field in the body of the document (without macros) and it gets unwieldy very quickly once there are more than (say) three selections e.g. with three selectiuosn ItemA, ItemB and ItemC
{ IF { REF Dropdown1 } = "ItemA" "This is the text associated with Item A" "{ IF { REF Dropdown1 } = "ItemB" "This is the alternative text associated with Item B" "This is the text associated with ItemC" }"}