First of all, which version of Foreman and Smart-proxy are you running?
So, as what I understand, you did create an BMC interface. Did you also fill in the correct ip-address for the BMC interface, correct provider setting (IPMI) and authentication settings? If all went well, from the host overview you’d also see the status in the upper left with some possible actions + a new BMC tab would appear.
Now, if the happy path doesn’t work, I’d like to ask to either observe the logs and look what it says after you’ve configured your BMC interface, or paste the relevant logs here after which we’ll be better equipped to help you out figuring this one out.