Drop down list in module builder backend?

I am new to Sitefinity. I already have created module with module builder. I have a custom field that will list pages in a drop down list. The selected page needs to persist its GUID. If I select type GUID for the custom field, I can enter the "Type or Virtual path of the custom widget". I've tried to create a user control, but I get a runtime error saying IField interface needs implemented.

What is the best way to handle this? I can't find a way to use a control designer w/ content created by module build. Perhaps a custom module would be better?

Any ideas are welcome.

I have the same error when I implement a custom field for news. I´m doing a property called section. To enter it I made a custom control wich consist in a textbox and a dropdownlist. In the dropdownlist you can choose a section from already existed but if you write something in the textbox a new section is created instead choosing the existing one.

I have searched for that interface and it is something for databind controls, in fact your module and mine have in common the dropdownlist. Still I haven't found the answer but if I find out I will tell you.

Has anyone had any success with this problem? I am trying to do something similar to the original post and documentation seems pretty thin so far.