VBA doesn't directly access tables. You would do this by creating an update query that selects the record you want to update if the value is null and sets the value to whatever you want.
I'm guessing that you are making this harder than it needs to be. Are you not using a bound form?
Microsoft Access
Microsoft Access is a rapid application development (RAD) relational database tool. Access can be used for both desktop and web-based applications, and uses VBA (Visual Basic for Applications) as its coding language.
ASKER