That tool Krk linked to is good for figuring out which parts you need to assemble an ET200S station.
To set it up in STEP7 HW Configuration:
1. Select a DP capable port on the CPU and set it to "networked".
2. A Profibus network will appear in the overview.
3. Select the Profibus network (it becomes solid black)
4. Open the HW Catalog and browse to the Profibus DP folder and then the ET200S folder. From there doubleclick an IM151-1 so that it gets attached to profibus network in the overview.
5. Select the new ET200S rack in the overview, and then the 1st slot.
6. From the HW catalog, find a power modules (from ET200S .. PM) and doubleclick so it gets inserted in the 1st slot.
7. Add i/o modules in all the other slots to suit your needs.
8. Save and compile.
There are plenty of settings and properties to play around with. But for starters just accept all the defaults.
Notice that the STEP7 HW Configuration does not include the terminal blocks. You can use the other link to find all the necessary accessories.