You have to define the arguments server side. 
Client side: 
triggerServerEvent ( "onLogin", localPlayer, username_editbox, password_editbox ) 
 
Server side: 
function login_seq ( username, password ) 
    local username = mysql_query ( handler, "SELECT account_name FROM account_info WHERE account_name = ".. username .."" ) 
    local password = mysql_query ( handler, "SELECT account_password FROM account_info WHERE account_password = ".. password .."" ) 
    if ( username and password ) then 
        spawnPlayer ( client, 1687.7318115234, 1448.0688476563, 10.768267631531, -90, 0, 0 ) 
        setCameraTarget ( client, client ) 
        setElementDimension ( client, 0 ) 
    end 
end 
addEvent ( "onLogin", true ) 
addEventHandler ( "onLogin", root, login_seq ) 
 
P.S: You also had a missing 'end' to close the 'if' statement.