DB2 connect is the product that will support access if db2 mainframe from db2 on unix ..
You'll have to license db2 connect ... If you run version 8 ESE of db2 on unix, five user licenses are included ...
And, again, how you use it, ie , as a federated database or a using direct application calls to the mainframe is dependent on your application requirement ...
HTH
Sathyaram