In InfoPath data source (main) create a Group . Then add a Text Field below this group and mark that as repeating. Now bind the Multiple selection list box to this text field.
That will fix the above error.
Now if you need to access this field in the code --> From the Data Source, right click on field -- copy XPath. Use this Xpath in the code to get access
Example:
this.CreateNavigator.Select("xpath....", NamespaceManager);
This returns an interator which you can use in a loop to get the values.
(Please check the syntax above for the code)