if you don't have a "transfer from as/400" button on your toolbar, or the option under the "data" menu, you need to install the add in. see this link for how to do this:

http://midwareservices.com/tipstricks/excel.htm

if you're trying to upload a file from excel to the as/400, its a little more tricky. you can save the file as a space-delimited and move the file to the as/400 ifs, then write a program to parse and edit the data.

you could also use odbc by setting up a table to point to the file on the as/400 and using something like access to edit the data. one of the big problems with importing from excel though, is insuring data integrity.