Hi,
The syntax for querying by a custom field is different – you need to set ‘meta_key’ and ‘meta_value’.
See the MapPress documentation for some examples, or here for the WordPress codex:
http://codex.wordpress.org/Class_Reference/WP_Query
I think your query will look something like this:
[mashup query="post_type=church&fellowship=rpca&meta_key=state&meta_value=GA"]