In the /horde/imp/config/conf.php file there is the following option:
// Should we display a list of servers (defined in config/servers.php)
// for users to choose from? The options are 'shown', 'hidden', and
// 'none'. If the server list is hidden then you can use the
// 'preferred' mechanism to auto-select from it based on an HTTP
// virtualhost or another piece of data. If it is shown, the user will
// be able to pick from any of the options. If none, no server list
// will be shownand the defaults will be used unless another
// mechanism changes them.
$conf['server']['server_list'] = 'none';
you should have:
$conf['server']['server_list'] = 'shown';
Additionaly you will need to configer the server settings in the horde/imp/config/servers.php.