P.W.
I am no expert by any stretch but I believe First you'll need to know what table your screen is based on.
Then...
Go to that table in CUSTOMIZE, TABLES/FIELDS and click "Edit Fields"
Next
Add a new field as a DROP DOWN in the exact name an punctuation as the table that has the fields you want to pull into your new screen.
(Example; If you're screen is based on the "Activity" table and you want to pull in fields from the "Cases" table you'll need to add a new field as a drop down called "Cases" in the "Activity" table)
Next
click "Add A linked Field" at the top of the same Edit fields Screen you're on
Then
Choose the "Link Using" field that corresponds to the table you just added as a drop down (from the example above it would be "Cases")
Then
Choose the "Linked Field"(s) from the drop down you want to pull into your new screen and repeat for each field you want to add.
Once you have added all the fields you want to pull into your new screen click "Finished Editing Fields"
You should now refresh your screen or log out and back into method before going to Customize the screen
Hope this helps.