You cant directly access the Database from Flash. You will
need to use server-side scripting, such as - ASP, PHP, ColdFusion
etc.. If you know any of these and if your server supports it, it
should be easy. Otherwise, you have some handy tutorials
you can learn some.
There are many ways to populate the Combobox from a script.
You can use additems one by one, use XML or something like Flash