Hello Murtaza;
Step 7 software can only communcate with the S7-300 or S7-400 family of PLCs, not the S5 CPUs.
Siemens has a software named Step 5 to communicate with S5 PLCs.
Other providers have developped software to program S5 PLCs, such as IBHsoftec, who propose S5 for Windows; here is a link to their download page: http://www.ibhsoftec-sps.de/english/Downloads_Demos.htm