How do i add a symmetry to my box model in 3dsmax where when i move a vertex,edge,polygon etc on one side the same thing happens on the other side instantly.  ive seen this done in zbrush and xsi wondering how i can do this on 3dsmax.  ive used the symmetry modifier but the other side does not move when i edit the other. thanks

You have to remove the poly (or polys) along the Axis of Symmetry.

Create a Box, convert to Poly, delete the poly where the middle of the final object will be, add the Symmetry modifier. Note that depending on the orientation of the model and the location of its Pivot Point you may need to move the Symmetry Modifier gizmo into position and maybe flip or change the mirror axis to get the results you want.
Also make sure that Show End Result is on or the results of the modifier will disappear when you drop into the Editable Poly sub-object levels to make changes.

Max 4.2 through 2013.
XP-64 (SP2)
NVidia 9800GTX-512 (Driver 266.58).
Core 2 Quad Q6600 2.4GHz, 8Gb Ram, DX9.0c.

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Don’t forget to weld your vertices when your finished along your symmetry line

Windows 7 64bit - 3dsmax 2012
Intel Xeon CPU W3540 @ 2.93GHz
GTX 580
GTX 460 for secondary monitors
12GB Ram
150GB x2 Velociraptor RAID BOOT Drive