I have loaded details of a customer on a list and was able to load the data with out any problem using sharepoint list destination. We have a lookup column called customerid looking at customer list I just loaded . This lookup column is in a list, called Customer Main, so I am trying to load additional data for customer in this customer Main  list which contains a lookup column called customerID. I was not able to load data into this cutsomerid column, whereas rest of the data is coming into sharepoint with out any problem. How should I load data into a lookup column. Please advise.

Thanks