I have gone through the "Add Row (DOM) with onClick Function " where u are adding textfields/checkboxs in cells.
I need to add dropdown menu with some values.
put the select and set its style to diplay:none.
assign the select an id
put its initial options
onClick call a function which does the following:
1)clears the select options (set the length of the select to 0)
2)adds new options to the select
3)sets the select's style to display:block
in this function you get hold of the select using document.getElementById("id");
(id is the id you have assigned to the select)
note if the options do not change everytime just do 3.