ShellApp sounds like a probable suspect.

Concerning file path separator there is a built in character
Application.PathSeperator

Also, Mac does not support ActiveX.
StrReverse??? I'm away from my Mac at the moment, so I'm not sure.

Excel VBA 2011 is a fairly big advance from 2004 VBA, so I'm not as confident about its features as I am about 2004. (If you are going cross platform, you might want to write for 2004 unless you are sure that all of your users have upgraded to 2011) (Excel 2008 did not support VBA, so....)

It might be best if you posted questions about specific issues as they arise.

The only other general advise I could give is to use conditional compiling
[VBA]#If Mac Then
Rem code for Mac
#Else
Rem code for Windows
#End[/VBA]

I hope this helped.