I think you need some more info.
I have a global field that shows the current serial number.
Why is this necessary?
On another layout and table, I have the ability to add to this database. When I add to the database from this layout, I want the new serial numbers to be created based on the global field showing the current serial number.
What are you adding, another record? How are these records related, if at all?
Can someone tell me how to add a serial number starting with the value in the global field that shows the current serial number?
If you answer the first 2 questions, someone can help you figure out your last question, but I really feel this whole concept is incorrect.
I don't see a reason for the global field. You can set up a field to auto-enter serial numbers and not need any starting value in a global or any other kind of field.
Using a global field like this will have two serious problems:
In a multi-user database where the database is hosted from another computer, Changes to the global value by one user will not be visible/accessible to other users and the value of the global field will revert to its original value when the user closes the file.
See this knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/3604/kw/global
In a multi-user database, there is a very real chance that two users creating new records at the same time will get new records with matching instead of unique serial numbers