Modify the updateDisplay and onBlur methods. The methods must call the immediate parent class of MultiSelectTextField. So Ext.form.ComboBox.superclass.onBlur.call will become Ext.ux.MultiSelectTextField.superclass.onBlur.call and similarly for updateDisplay.
One feature I'd like to request is the ability to have items entered in the field that arent in the store.
For example, think of a tagging system (or an email to field). You can select an existing tag (or email address from an addressbook), but you can also use your own tag (or enter a freeform email address that isnt from the addressbook).
Also, in the example, I dont see a way of querying the underlying selected values. Is the right approach to query the form for the hidden fields and get the values that way? Any chance of a method that does that work for you? (could just return this.values()!)
Would be a nice addition to the demo page, to have the send button pop a message box with the current selected values.
Finally, is it possible to have the separator char also act as a 'select' trigger for the dropdown? So as well as enter selecting the highlighted item, the separator would do the same.