Hi Lee,
AutoCompleteExtender can work fine in a UserControl, but you cannot populate it from a page method, page methods only work in pages not in UserControls so you have to use a .asmx web service to populate it from an url.
So you have to implement an .asmx with the service method and then set the ServicePath="~/path/to/yourservice.asmx" on the AutoCompleteExtender
Hope that helps,
Joe