it works; problem is probably with using an optional filefilter. alternative methods of using filefilter for the mac are documented in the VBA mac help file. While not explicitly stated, I'm pretty sure wildcards do not work on the mac (please correct me if I am wrong).

SO, you can either get rid of the filefilter, or do something like:
[vba]
If RunningOnMac = True Then
FileName = Application.GetOpenFilename(Title:="Browse for file")
Else
FileName = Application.GetOpenFilename("Excel files (*.xl*),*.xl*", _
1, "Browse for file", , False)
End If[/vba]

as you can see, I opted here not to deal with the filefilter on the mac, but did use it in windows.