Outlook doesn't support exporting custom fields. Quick and dirty method:

1. Put all the items from your form in one folder.
2. Use a table view to display all the data fields that you want to export
to Excel.
3. Choose Edit | Select All.
4. Choose Edit | Copy.
5. Paste to Excel.

Otherwise, you'd have to write custom code or use a third-party application.
See http://www.slipstick.com/dev/customimport.htm.