To my understanding, you can add files directly from MacVim with the NERDTree plugin. I navigate to NERDtree and hit 'm' to bring up the NERDTree filesystem menu as described in this post: vim and NERD Tree extension - adding a file
The problem is, I get this output after invoking 'm':
NERDTree Menu. Use j/k/enter and the shortcuts indicated
==========================================================
Error detected while processing function <SNR>14_showMenu..30..31:
line 4:
E716: Key not present in Dictionary: menuItems)-1)
E116: Invalid arguments for function len(self.menuItems)-1)
E116: Invalid arguments for function range(0, len(self.menuItems)-1)
E15: Invalid expression: range(0, len(self.menuItems)-1)
Press ENTER or type command to continue