Hi there, and welcome to the forum!
I assume that you're looking to connect to the database using VBA? I have an article on my site to retrieve data from a database that you can find here. Be aware, however, that the article connects to an Access database.
The methods should be very simliar though, but you would need to update the connection string to something that is compatible with the as400. I found some connection string for the as400 here that may work. (I can't test that part as I dont' have access to an as400 system.)