I'm very bad explaining things, but I will try it anyways
First of all, set your Main pivot point where you think the CG (Center of Gravity) of your airplane is. Then open "c172r.xml" and edit the following code:
<location name="CG" unit="IN"> <--- You also can use [unit="M"] to use meters instead of IN(inches)
<x> 39 </x>
<y> 0 </y>
<z> 36.5 </z>
</location>
Set X,Y,Z to "0", so the CG will be where your Main pivot is. (If I remember well, I had a problem here with X axis, so I set it to 1.0 [<x> 1.0 </x>]).
I also used to set all X, Y, X of <location name="VRP" unit="IN"> and <location name="EYEPOINT" unit="IN"> to 0.0
Then you scroll down and under <ground_reactions> and you have:
<contact type="BOGEY" name="NOSE">
<location unit="IN">
<x> -6.8 </x>
<y> 0 </y>
<z> -18 </z>
</location>
<static_friction> 0.8 </static_friction>
<dynamic_friction> 0.5 </dynamic_friction>
<rolling_friction> 0.02 </rolling_friction>
<spring_coeff unit="LBS/FT"> 3800 </spring_coeff>
<damping_coeff unit="LBS/FT/SEC"> 400 </damping_coeff>
<max_steer unit="DEG"> 10 </max_steer>
<brake_group> NONE </brake_group>
<retractable>0</retractable> <------ Set this to 1 if your landing gear is Retractable.
</contact>
This is the NOSE wheel (you can add many wheels you want, by duplicating the <contact> section renaming each under name="NOSE" and obviously write the correct coordinates).
You have to write the coordinates of each wheel under X, Y, Z. Note: The coordinates are exactly the same as in 3ds Max, except In Max, Y Negative == X Positive in FDM I think in Blender in the same, if not, use trial and error, that always works
Note: All coordinates are relative to the Main Pivot point.I hope you understand my poor explanation
