Well, way late, but I found this looking for the answer myself before solving it, so I’ll post for others in my boat, as I’m sure the OP sorted this by now. 😉
Use "meta_key" and "meta_value" in your query after taking a look at “post_meta” in your database to figure out what the simple fields plugin created, like the OP did above. The query I needed which is posted below grabs posts from 3 different post types (2 custom) where the simple field I added: “feature in homepage slider” has a dropdown value of yes (meta_value: dropdown_num_3).
query_posts(
array(
'numberposts' => '10',
'post_type' => array('filmp', 'corp', 'post'),
'meta_key' => '_simple_fields_fieldGroupID_1_fieldID_3_numInSet_0',
'meta_value' => 'dropdown_num_3'
)
);