There's a pretty good tutorial about creating a front-end search for custom fields here:
Maybe that will be of help to you?
*Aah it may be a bit out of date as it's a couple of years old, someone in the comments says this, maybe that would be helpful instead:
you shouldn’t be using query_posts.
Here is a better method that uses the pre_get_posts filter:
since WordPress 3.1 you can use the meta_query parameter to search by custom field values.