Please see the help about User Defined Fields, The User Defined Field(s) must exist in the Outlook Folder before starting the assign process.
GeniusConnect will use the first Item in the folder to collect the user defined fields.
If your folder contains items without user defined fields:
1. Move the items temporally to different folder.
2. Create a dummy item with user-defined fields (also assign dummy values to the user-defined fields).
3. Go to GeniusConnect setup->Database and complete the assign table process.
4. Remove the dummy item
5. Move your items from the temp. folder to your folder