There are 5 Hilton hotels around Paris. I'm assuming that by 'in Seine' you mean the one closest to the river (i.e. Hilton Paris, on Avenue de Suffren, right next to the Eiffel Tower).
From Gare du Nord, take RER line B, southbound. (The train's destination will be either Robinson or St Remy-les-Chevreuse). Alight after just 2 stops, at St-Michel Notre-Dame.
Transfer to RER line C, westbound. (The train's destination will be either Versailles-Rive Gauche or Saint-Quentin-en-Yvelines). Alight after 4 stops, at Champ de Mars Tour Eiffel. The station exit signs might direct you to Avenue de Suffren. If not, just follow the crowds heading towards Tour Eiffel. You'll exit the station with the Seine on your left and the Eiffel Tower straight ahead. Avenue de Suffren is the road off to your right, immediately in front of the Eiffel Tower lawns. The hotel is a few metres along the road.
Chris