Hi everyone.

When trying to unfold my model's UVs, I get this error:

Roadkill kind of works... I find the plugin pretty hard to work with for this model so I'd love to get the standard Unfold working. But if I can't then Roadkill will do I suppose. Any ideas what's causing this?

I'm not too familiar with Unfold, what kind of issues are you having with Roadkill?

I guess I'm not as comfortable working with it and I'm having a hard time selecting the right edges in it because the model is fairly high poly. I've watched a few tutorials and understand the basics I think.
Do you know of a way in XSI to symmetrically select components? Say I want to make a cut line for each arm but my edge loops aren't very clean so I'm having trouble making sure I select the same edges on both arms... Is there a way to select an edge on one arm and then the "mirror" edge on the opposite arm will also be selected? That would make this much easier and I can't seem to find a way to do that.

I rarely do anything inside Roadkill, I mark all my seams or disconnect edges in XSI and just let Roadkill do the bulk work.
I have "Use Roadkill interactively" unticked in the plugin so it does a quick "send to>unwrap>send back", then I fine-tune the UVs in the Texture Editor.
I don't know a way to select edges symmetrically and simultaneously offhand, but you can easily select edge loops with alt+middle click or select a range with alt+left click.

So I tried using Roadkill, and I did it several times with the same result... all the UVs end up smushed into one point in the bottom left of the UV map... no matter which edges I select.
Edit: forgot to mention this is the same result as I got with the default unfold.

Does that happen even with a cube?

EDIT
It looks like mSymm might be able to mirror selected edges, you can see in the demo video at 2:30 it works with polygons:
https://vimeo.com/39883619

The author has that part of the tool titled "Mirror Selected Components".

EDIT2
Yep the tool will mirror point, edge or polygon selections, you must have a symmetry map on the object.
This plugin was designed to work with version 7 or higher but it does not work with Mod Tool 7.5

The issue does not occur with a cube or other primitives (I tested a cube and a cylinder).

Thanks for pointing out that plugin, I'll give that a try.