Make mysql query use an index

gistfile1.sql

/* I want to run this query: */

SELECT SUM( price ) AS total_price, date_month
FROM daily
WHERE processed =0
AND date_month = '200910'
AND customerid =1
GROUP BY date_month, customerid

/* On this table */

CREATE TABLE IF NOT EXISTS `daily` (
`id` int(10) NOT NULL auto_increment,
`error` varchar(255) NOT NULL,
`customerid` varchar(10) NOT NULL default '',
`dateDay` date NOT NULL default '0000-00-00',
`price` int(10) NOT NULL default '0',
`ourcalculatedprice` int(10) NOT NULL default '0',
`processed` int(1) NOT NULL default '0',
`date_month` int(6) NOT NULL,
PRIMARY KEY (`id`),
KEY `customerid` (`customerid`,`date_month`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;

/* 
How can I make it use an index. Right now it is really slow. I added date_month just
to make the query use this index.

What I need is the SUM(price) for a customer on one month where the amount has
not already been processed. As soon as the amount is processed it will be set to 1.

How can I make the query use indexes?
*/