I'm assuming that mod that you're using doesn't change the files that this control uses, if that is the case then all you have to do is replace these files:
ProductAttributes.ascx.cs - from "Modules" in "NopCommerceStore"
AttributeControlTypeEnum.cs - from "Libraries/Nop.BusinessLogic/Products/Attributes
and add NopPrefilledDropdown.cs to the Libraries/Nop.Controls project and then recompile your code and upload the .dll files that have been modified.
If the mod you're using does use the files that are modified with this new control then you'll have to open up the files and figure out what code to bring over into your files.
ProductAttributes.ascx.cs - from "Modules" in "NopCommerceStore"
AttributeControlTypeEnum.cs - from "Libraries/Nop.BusinessLogic/Products/Attributes
and add NopPrefilledDropdown.cs to the Libraries/Nop.Controls project and then recompile your code and upload the .dll files that have been modified.
If the mod you're using does use the files that are modified with this new control then you'll have to open up the files and figure out what code to bring over into your files.
This didn't work I'm afraid :o(
I replaced my files with yours and when I "Build Solution" I was getting errors. I don't know much about VWD but am sure that errors in the build can't be good! I replaced the .dlls just in case and although the "Prefilled Dropdown List" option was there when I tried to add my attribute, the options I created, i.e. colours and sizes were not. The dropdown list was empty on my product page.
I also took the original downloaded 1.8 source files and tried with this but it doesn't look like anything gets updated. When the site launched there was not even an option in the product varients for a "Prefilled Dropdown List".
This is a shame as I would really love to use this mod.
If you do ever happen to create this again for v1.8 (for whatever strange reason) I would really love a copy of your files.
Thanks for all you advice on this...
Glenn.