LinqPad custom plugin: extension method not found

Hi, first time poster here.
I've been trying to write a simple plugin for LinqPad (currently running v4.45.05 PRO), but LinqPad doesn't detect my extension methods.

I've written a simple VS project, which references LinqPad.AnyCPU, with one class which looks like this:

public static class Extensions
{
    public static void ShowCustomVisualization(this List<double> valueList)
    {
         var c = new CustomDisplay(valueList);
        PanelManager.DisplayWpfElement(c, "CustomDisplay");
    }
}

This project is built in Release mode, and copied to My Documents\LinqPad Plugins, which was created by LinqPad.
The project references another dll, which contains some logic for displaying, and that dll is also copied to the Plugins folder.

However, when I write a simple piece of code like this in LinqPad:

var d = new List<double>{1,2,3,4,5,6,7,8};
d.ShowCustomVisualization();

It does not find the extension method.

Yes, I've tried "turning it off and on again" :-) I've also tried to create different signatures for the extension method.

Doe anyone have any idea what I'm doing wrong?
Thanks!