Omarrosa,
with the PG720 came a standard package with the bag, manuals, power supply, and an MPI-cable . If you havent mislaid it, then you have it allready. If you cannot find it, then get a straight cable like the one PLucas described.
The cable plugs into the MPI ports on the PG720 and the S7 CPU without any further devices in between.
edit: and its a common misunderstanding that MPI is limited to 187.5 kbaud on the MPI side. MPI works up to 12 Mbaud. some older devices are limited to 187.5 kbaud or 1.5 Mbaud.