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.
If some of the fields are multi-line, see Eric Legault's handy tip for an Excel formula to break up the lines -- http://blogs.officezealot.com/legault/archive/2005/04/08/4502.aspx
Otherwise, you'd have to write custom code or use a third-party application. See http://www.outlookcode.com/d/customimport.htm .

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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
http://www.turtleflock.com/olconfig/index.htm
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
>I have additional fields for my contacts under the user-defined fields, but
> when I try to export to excel, they don't show up under the mapping. How do
> I get them on my spreadsheet?