Go to directory where TextWrangle keeps all the scripts:
$ cd ~/Library/Application\ Support/TextWrangler/Text\ Filters
Create a new file for XML formatting:
Text Filters $ touch FormatXML.sh
This will open the new file in TextWrangler (or whatever editor you have set).
Text Filters $ edit FormatXML.sh
Enter this text in that file and save:
#!/bin/sh
XMLLINT_INDENT=$'\t' xmllint --format --encode utf-8 -
Now you can format any XML by selecting TextWrangler menu:
Text > Apply Text Filter > FormatXML