You’ll also need to specify &tplType, which takes a value of either embedded (the form is on the page, and the script merely sets the placeholders), modChunk (which takes a chunk’s contents, you’ll want this), file (gets the content of a file containing the template) or inline (also the default: the template is within the snippet call).
This works for me:
[[!Login? &tplType=`modChunk` &loginTpl=`loginTpl2`]]
Edit:
I’ve updated the documentation and the example at
http://svn.modxcms.com/docs/display/ADDON/Login+Snippet to show the parameter and make it display the proper default value.