when i was compile with -Xlint: unchecked then got some warning like this

Compiling 12 source files to C:\Users\Titus\Documents\NetBeansProjects\SITA\build\classes
warning: [unchecked] unchecked call to addItem(E) as a member of the raw type JComboBox

what should i do?

either adapt your code, or use it as is. it's a warning, not an error.

if my code like this then? how to solve warning?

 private void KDM (){
         jComboBox2.addItem("-- pilih menu --");
          try {
            String sql = "select * from iomenu";
            Statement cn = con.createStatement();
            ResultSet rs = cn.executeQuery(sql);
            while (rs.next()) {
        } catch (Exception e) {

Stop posting the same question twice. Next time you will get an infraction, possibly leading to a complete ban.
Read the DaniWeb rules carefully before you post again

And yes, as people have already pointed out, there's no point trying to work with GUIs and databases when you seem to have skipped over the basics.

and the basics include learning how to read and interpret messages from the compiler.