I had a similar problem with our old Thermwood router. What I found...
In the mastercam help wile there is a sample of a 3 axis post, and it shows how to invert the axis. I assume that you are modifing an existing post processor to work with your machine.
Make backup copies of your post before you start messing with it.
Open it with any text editor and find the Motion control section, add a line to the section that writes the line for the axis. Something like this..
# --------------------------------------------------------------------------
# Motion NC output
# --------------------------------------------------------------------------
##### Custom changes allowed below #####
prapidout #Output to NC of linear movement - rapid
xabs = xabs * -1 # <------Invert axis output
pcan1, pbld, n$, `sgcode, sgplane, sgabsinc, pccdia,
xout, yout, zout, s_out, p_out, strcantext, scoolant, e$
plinout #Output to NC of linear movement - feed
xabs = xabs * -1 # <------Invert axis output
pcan1, pbld, n$, `sgcode, sgplane, sgabsinc, `sgfeed, pccdia,
xout, yout, zout, s_out, p_out, `feed, strcantext, scoolant, e$
if nc_lout$ <> m_one & feed = zero, psfeederror
Your post might be different but hopefully you get the idea. Double check your g-code output to make sure things are moving in the right direction. And double check the g02 and g03 you may have to add that line in another place for those.
From what I have heard, the newer versions of Mastercam are going to control the options for the post through the control definition file in Mastercam itself. I have an older version with an even older post so it doesn't do much for me, I have to manually edit the post.
Hope this helps