If your Excel/csv file does not contain account information, please follow the steps here.
Setup and add accounts to the accounts table
From the Settings tab > Transmission Options > Transmission File Format tab
--Select 'Multiple Accounts' option (radio button), and then
--click the link to Manage Multiple Accounts
Upon clicking the link to Manage Multiple Accounts, you'll see the following window.
Click 'Add' and then enter an account nickname (used internally only) and the full account number.
Repeat for as many accounts are needed.
Close both windows when complete.
During the import process, you'll be able to associate each file with an account.
At the column mapping screen, there will now be a drop down selector for the account. Select the account during import and then continue as usual.
Create the file as usual.