Your question is quite wide as there are many ways to communicate with an AS400.
Essentially if you want to access a database table then you can use any number of tools e.g. Microsoft Access ODBC tools available with Microsoft Access and send SQL queries to the database. NI have a database {NI} toolset or you might look at the free LabSQL toolset here {http://www.jeffreytravis.com/lost/labsql.html} requires MDAC 2.6 {Microsoft} which is distributed without charge, I think there are later versions available but you should check out bugs / issues. There will need to be support for ODBC connections installed and enabled on the AS400 and a license may be required for this depending on the application running on the box, it depends on the supplier.
I suggest that you create a prototype database hosted on another box (perhaps in MS Access) and run everything against that for a while before going live! Your system may need to account for the LONG delays that you will experience talking to an AS400. I spend most of my effort managing connections and data validity as they love to take them down (AS400s) without much notice, some of this can depend on the exact installation, my self I have load of experience working accross long links, time zones and the problems they create.
What you will need in any case is an account on the AS400 and how you handle that account will no doubt bother IT immensely. They will probably be most uncomfortable if you try to cache access information. Also you may need or have to be given far more access than they would like, to the table that you intend to use. After SOX (Sarbanes-Oxley {wikipedia}), they will no doubt have even more excuses for giving you a hard time if they are attempting to become SOX compliant any time, as no doubt the AS400 in question is probably run by accountants for bean counting.
Alternatively of course you could be trying to use a telnet session or an RS232 link or...... well I hope I hit the nail on the head first time.
