witch line was it that threw exception?
1st Problem: if statement is incoret, when you compare you must use == instead of a single =. Like this:
if (tt == true)
{
}
2nd Problem: FindByText returns as ListItem instead of Text. Try this:
bool tt = Convert.ToBoolean(DropDownList1.Items.FindByText(TextBoxNew.Text).Text);
ps: I am not behined Visual studio so i do not know the accuarty of the answer