Tried:
adt -package -keystore HelloWorld.p12 -storetype pkcs12 -target bundle HelloWorld.app HelloWorld.xml.
And then it complains about the . at the end:
File /Users/cameron/HelloWorld/HelloWorld.xml. does not exist
So it looks like it's looking in the right place fine, but doesn't like the XML file for some reason... or the command is expecting an AIR file rather than an XML file...