If it is a factory fresh new CPU with no program:
Set an suitable IP address in the HW Configuration.
Then load the HW Config in the CPU.
When the dialog suggest as target for the download the IP address you have set manually, you must instead search for the CPUs MAC address, or type it in manually (the MAC adress is printed on the CPU). When the MAC adress is correctly specified, you then download the HW Config.
A dialog will pop up and ask if the IP must be set on the CPU, and you just say yes to that.
From then on you can reach the CPU by its IP address.
Notice that your PC must have an IP address in the same IP adress segment as the S7 CPU.
If it is an existing CPU with an already loaded program:
You must have a backup with the proper IP address set in the HW Config.
Notice that your PC must have an IP address in the same IP adress segment as the S7 CPU.