In order for the “paste opposite” button to work your mesh needs to meet some specific criteria:
It has to be symmetrical across the X-axis. If you go into Edit mode on your armature and press “1” on your number pad, are you looking at the front or back of your model? If not then your model is not symmetrical across the x-axis. When I do this to your model, the guy is looking toward the left side of my screen. He is not symmetrical.
The left and right side bones need to have the same name but with a “.L” and “.R” denoting left and right. Your feet are called “RFoot” and “LFoot”. They probably need to be called “Foot.R” and “Foot.L”.
Other things that are messing you up are that your model and armature both have locations and scales applied to them. These should be ‘applied’ to the mesh/armature because they can cause problem down the road.
Thanks. I’m going to have to look into rigging properly when I have the time. I’m trying to follow the MFBGE tutorials and rigging is a small part of it. Shame it’s hard to get a grasp on compared to everything so far…