I would intially try to telnet in with the Terminal. I used to do this regularly with an AS 400. Enter telnet and the ip addess of the AS400, you should get the login screen.
Otherwise, there is a pacakge from NLynx named macmidrange that is specifically for this. Years ago I used the predecessor of that software to link to an AS400 when IBM said it coudn't be done. You should have seen the espression of the salesman's and SE's face!
HTH
Craig