Ok, if you REALLY want to get to Carrefour by public transport....
Train to Mulhouse
Tram number 1 from station to Porte Jeune
Tram number 2 from Porte Jeune to Nouveau Bassin
Bus 22 from Nouveau Bassin to Centre Commercial.....and there you are
But, adrianlondon is right....Geant in St Louis is MUCH easier and closer, or you can also go to Leclerc, which others have mentioned early on...you can take the French 604 bus there from Schifflande, but be warned, the bus only takes euro coins now and will cost you 1.10 each way.