Try running this query:
CREATE TABLE `wp_useronline` (
`timestamp` int(15) NOT NULL default '0',
`userid` int(10) NOT NULL default '0',
`username` varchar(20) NOT NULL default '',
`displayname` varchar(255) NOT NULL default '',
`useragent` varchar(255) NOT NULL default '',
`ip` varchar(40) NOT NULL default '',
`location` varchar(255) NOT NULL default '',
`url` varchar(255) NOT NULL default '',
`type` enum('member','guest','bot') NOT NULL default 'guest',
UNIQUE KEY `useronline_id` (`timestamp`,`username`,`ip`,`useragent`)
)