Thansk for your help, I still have a little problem. After I get the
message do you want to add it to the list and if I select Yes, nothing
really happens. What is wrong with my code; did I miss anything? I
don't understand how it knows to what named range it should add the
new value.
I have:

Sheet1 D3 has Validation 'Product'
Params C2 'Product' = columname
Params C3 'Product item 1'
Params C4 'Product item 3'
etc.

I have added the named range 'Product' = =OFFSET(Params!$C$3;0;0;COUNTA
(Params!$C:$C )-1)

I think I have to change this piece: Worksheets("Params").Cells
(Rows.Count, 1).End(xlUp).Offset _
(3, 3).Value = NewEntry