Hi Jen.  I just tried this and it worked for me.  Can you see if you missed any of these steps?
1.  Go to the Customize tab.  Click on the Tables/Fields tab link.
2.  For the Customer table click on the Edit Fields...
3.  Add a new field that has a Field Type of Text and a Size of 4000 characters.  Called it TagList.
4.  Customize your scren.  In the list of fields in the Add Fields tab on the left you should now see a field called TagList.  Add this to the screen.
For #4 you need to add the field TagList.  Adding a textbox from the Add Objects tab and naming it "TagList" will not work.