Thread:Extension talk:User Merge and Delete/Help: "Fatal error: Call to a member function isListed() on a non-object"/reply

hmm, what version of MediaWiki are you running, and which version of this extension?

If you're running a recent version of MW (1.13+) try downloading the development version from here:

http://upload.wikimedia.org/ext-dist/UserMerge-trunk-r93008.tar.gz