Hi Ken,
Since the field was added to the Activity table, it won't be available in your Invoice screen as it's based off of the Invoice table. What you can do is edit the Invoice table to create a Dropdown which is attached to the RecordID field of the Activity table. Once that's done, you can create a Linked field which points to the custom field in your Activity table. Then you'll be able to drag and drop that field on your Invoice screen using the screen designer.
If you haven't already, I strongly suggest you watch our Customization Webinars (scroll down towards the bottom of the screen) in order to get a better understanding of customization. Alternatively, you can always speak with an in-house Consultant and pay for an hours worth of training. This option will probably be the most beneficial to you if you plan on further customizations.
Let me know if you would like to speak with one of our in-house Consultants and I'll get the ball rolling.
- Ashur