Did you try using Open URL to open a specified folder? That works for me in windows. Not sure that it works for Mac users...
I tried that first but it opens in Safari
Use the "Perform AppleScript" script step and use this script:
tell application "Finder" to open ("/Volumes/Macintosh HD/Path/To/Folder/" as POSIX file)
and what effect would that have ? Why invoke it from FileMaker ?
Put an alias to your Dropbox folder in the dock and click on it...