Matt,
Observium considers IPMI another component of its system, not a system in itself. So you have to add the actual system first.
Once that's up and running, I'd click the cog icon on the right side of fooserv's device page, click IPMI, and enter the IP address and login details for the IPMI board there.
Security note: If possible I'd suggest setting up a user on your IPMI board that has access to read sensors, but can't control the chassis or console, so that if your Observium host is compromised you don't have root-equivalent credentials in your database. However, not all IPMI boards allow this.