mvim (MacVim 7.4) opening new windows rather than tabs (Mac OS X 10.8.5)
use the --remote-tab command line switch, as in:

mvim --remote-tab foobar.txt


The answer was obtained by reading help:mvim it is located in section 14 of the help file titled macvim-hints.

Extracted here is the relevant section of the MacVim 7.4 help file:


  Scenario: 
  
  ~ You want to open a file in a tab in an already opened
   window, but typing "mvim filename" in Terminal opens it up in a
   separate window. 
  
  Solution: 
  
  ~ Use the |--remote-tab| switch.  If you
   have several windows open you might have to specify which window you
   want the file to open in by using the |--servername| switch.  The
   title of a window usually ends in something like "VIM" or "VIM3" ---
   this is the server name of that window.  So to open a file named
   "foobar.txt" in a window whose title ends in "VIM3" you would type
   (the order of the arguments matters):    
  
  mvim --servername VIM3 --remote-tab foobar.txt 
  
  For more information, consult the |client-server| manual page.