According to the README, this should work:

Microformat.find :all => { :text => 'mfin text' } 

However:

Adr.find(:all => {:text => '<div class="adr"><div class="postal-code">01234</div></div>'})
NoMethodError: You have a nil object when you didn't expect it!
The error occurred while evaluating nil./
        from vendor/plugins/mofo/lib/microformat.rb:151:in `find_occurences'
        from vendor/plugins/mofo/lib/microformat/simple.rb:12:in `find_occurences'
        from vendor/plugins/mofo/lib/microformat.rb:23:in `find'
        from vendor/plugins/mofo/lib/microformat.rb:282:in `parse_element'
        from vendor/plugins/mofo/lib/microformat.rb:226:in `build_hash'
        from vendor/plugins/mofo/lib/microformat.rb:202:in `each'
        from vendor/plugins/mofo/lib/microformat.rb:202:in `build_hash'
        from vendor/plugins/mofo/lib/microformat.rb:201:in `each'
        from vendor/plugins/mofo/lib/microformat.rb:201:in `build_hash'
        from vendor/plugins/mofo/lib/microformat.rb:155:in `build_class'
        from vendor/plugins/mofo/lib/microformat.rb:133:in `find_every'
        from (irb):1:in `inject'
        from vendor/plugins/mofo/lib/microformat.rb:132:in `each'
        from vendor/plugins/mofo/lib/microformat.rb:132:in `inject'
        from vendor/plugins/mofo/lib/microformat.rb:132:in `find_every'
        from vendor/plugins/mofo/lib/microformat.rb:28:in `find'

I think I have a fix for this; I just need to write a test.