This requires a Smart Proxy to be set up in first place (which is typically done right after you installed your Foreman) , like this:
Select Infrastructure => Smart Proxies
If your Puppetmaster is living on the Foreman instance (typically the case) , you specify the FQDN of the Foreman including the port for the Smart proxy, usually 8443. If the Smart Proxy is located on another system and / or port, adjust accordingly.
Once the Smart Proxy is set up, you will also see new hosts, at least the Foreman instance it self.
Next, you are going to import existing environments from the Puppetmaster:
The Foreman will know where to look for Puppet modules. It will import any valid module it finds. If you are importing some modules and they do not show up after the import, there probably is something wrong with the module, i.e. some gross syntax errors etc.
See also our post about parameters in Puppet.