Kualan

Did you also confirm there are no duplicated names in either the Object, Object Data or Ghoul 2 boxes? The last one catches out a lot of people.

Check the .skin file again, that message displaying means the .skin file has listed a surface that the .glm doesn't actually contain. Use CTRL+F to search for it.
 
This message will display in Modview even if the surface is listed as *off in the .skin file. Just delete the relevant entry.

There are different kinds of .fbx. You need to use the converter to change it into one that will work with Blender (such as FBX 2013).

The names need to be changed in three places:
 
1. The name box in the Object tab
2. The G2 properties box further down in the Object tab (nb: The name entered here must be exactly as it will appear in the .skin file)
3. The name box in the Object Data tab.
 
1 + 3 should match exactly, and avoid capital letters. It can lead to issues with Modview.

When you mirrored the arm mesh and renamed it to "l_arm" or whatever, did you also rename the bones? It looks like at the moment that the mirrored arm might still be linked to the original arm's bones? 
 
Go to the Object Data tab and under Vertex Groups check the names of the bones. E.g. if the mirrored arm is supposed to be the left arm, make sure under Vertex Groups it doesn't have bones like 'rhumerus' 'rradius' etc. If it does, just double click the entries and rename them to the exact same but with an L instead of an R at the beginning ('lhumerus', etc).

I can save you the trouble and point you to look up Jeff's thread in the WIPs subforum - this model is already there.

No, you don't need to go into the UV Image Editor. You can do all this in the main window by using the Space key to produce a dropdown menu that lets you type-and-click commands like Seams From Islands or Select Similar.
 
I'll try again, covering every keystroke:
 
1. In the main window, open Edit Mode. Press A to select all.
2. Press space and type in 'seams' - an option for 'Seams From Island' will appear in the menu. Click it.
3. All relevant edges will become red. Make sure you have Edge Select on only and right-click on one of the red edges.
4. Press Space. Type 'similar' and then click Select Similar.
5. In the next menu that appears, click 'Seam'. This will select all the red highlighted edges.
6. With them all selected, press Space. Type 'edge'. Click 'Edge Split'.
7. Check that the vertex count is still below 1000 for the surface and then export.

Typical! A problem plagues me for days and moments after posting this topic I find my answer!
 
For those also looking for the answer here, read the 'Advanced Usage' section of this ModView tutorial by HapSlash:
 
http://hapslash.tripod.com/jkii/modview.html