I work with Maya, and I don't know anything about 3Ds Max. Though, I do know how to do that in Maya, does 3Ds max have Duplicate Special?
If it does, goto Edit > Duplicate Special, and set it to mirror, and I think you have to position it where you want it. I don't know for sure, I just somehow know that answer. xD
[Warning] This could not be the correct answer, I'm not a pro and I literally just picked up Maya, this information is worth trying if 3Ds Max has that command. [Warning]