Carp spend most of there time somewhere in the water column depending on weather conditions, zigs allow you to try and fish for them in areas other than the surface or the bottom.
Pop ups will work as will foam, with foam it is more likely what it is made up of that makes them take it unflavoured (try listening to the first Frank Warwick carp cast), personally I have had success soaking foam in CC Moore;s Natural Blend (not available any more ), I think Nige soaks his in Essentials Thaumatin-B and I have friends that use Betalin (Both are similar sweet based products) as well with good success rates.