Update: I have activated WordPress SEO with my custom non-standard theme, and the meta tag function and title functions work well.
However, the XML sitemap feature appears to be broken. Where it says "You can find your XML Sitemap here," I click the button and get this error:
"This page contains the following errors:
error on line 2 at column 6: XML declaration allowed only at the start of the document
Below is a rendering of the page up to the first error."
Why would this be occurring?