1) You need to add you web service as Web Reference in your application where you want to use this web service.
2) Once you will add Web Reference, you will be able to access your web service class.
3) Create the object of your class.
4) by using object of your class access the function you want to bind with dropdownlist. Binding is similar as we are doing normally.
I just write a web service in which function return a dataset. Now I have load that dataset in asp:dropdownlist in a web page. Please let me know bow would I do this.