I have a meyer mpc that has one side rail thinner 
than the other...
what would be the best way to file the side rails 
to even them out. Any help would be greatly appreciated.

Uniformity here is not real important sound-wise.  You probably wont notice
a difference unless the window is made larger.  But it is an indicator of
the general quality of workmanship.

First center a reed on the piece to see if the outside of the rails are
wider than the reed.  If so, you probably should thin the wide rail from
the outside.  If the edges match the reed, you can thin the wide rail from
the inside.  I use a file that tapers to a point.  Then some sand paper
wrapped around the file working my way to finer grades and just finger
sanding.  More care must be taken as you approach the tip rail.  It is easy
to bugger up the corner.  I use a small blade from a pocket knife to work
on the corner if the MP material is soft enough.

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Thank you Keith.