The S7 protocol doesn’t support browsing, so tags have to be added by hand in the designer.
Create a new SQLTag, and for the Item Path you would specify something like “[MyDeviceName]MyAddress”, where “MyDeviceName” is whatever you named your device when you set it up in the gateway and “MyAddress” follows the address format for the kind of device you are connecting to.
The manual page for the Siemens driver describes the address formats a bit. Addressing a tag in a DataBlock would be something like:
[MyDevice]DB11,B0which would address DataBlock 11, offset 0 as a Byte.