I just wanted to post the problems I was having and what I did to solve them to help others who might be experiencing the same thing.
Getting the snippet to work as described was fairly simple, I just had to make sure I removed Rich Editor and change the file type on the "Page Settings" tab.
I had trouble with the XSL to show up on the sitemap. It didn’t seem to recognize the "sitemap.xsl" document alias. I found 2 solutions.
1. Create an actual file called sitemap.xsl and put it in the same directory level as your index.php page and assets folder.
2. Add the parameter &xsl=`sitemap.xsl` to your snippet call
I’m not sure if #2 did anything, but at either rate, when I did those things it seemed to work. I then tested to see if I could remove the physical file and just use the document in ModX, and that seemed to work.
My second problem, was not getting any results to show up. It said "Number of URLs in this Google Sitemap: 0" but when I viewed the source, all of my links were there.
After some searching online, I found that I needed to make a change in the code on sitemap.xsl
Around line 29, you’ll see the following line:
xmlns:sitemap="http://www.sitemaps.org/schemas/sitemap/0.84"
Change it to:
xmlns:sitemap="http://www.sitemaps.org/schemas/sitemap/0.9"
You may need to refresh a couple times, clear your cache on Modx or your browser in order for it to show up, but it should eventually.