In ACT! go to the TOOLS menu and select DEFINE CUSTOM TABLES
Click on the CREATE TABLE button
Enter the name of the custom table in the window.
Click on the SAVE button
By default here are a few system field which are automatically added to your new custom table. These fields include: Edit Date, Created Date, Completed, Last Edited By, Parent EntityID, Parent Entity Name, Record Creator and Record Manager.
Once the table has been created you can add you desired field by clicking on the Create Field link