RB ETH3 10.0.x.x/30 --------10.0.x.x/30 ETH 2 RB 10.0.x.x/30 ------- bridge to another building across the street --- 10.0.x.x/30 RB 10.0.x.x ----- 10.0.x.x/30 RB

I configured OSPF on the RB's.
Now when looking at the OSPF interfacces of the first RB, there is no instance or area for ETH 3 which is connected to the second RB.

How can I add instance and area to an interface in the OSPF section?

You don't. Area is determined by ospf network configuration and instance is determined by area configuration.

Thank you for your response.

But I don't understand why not?
How can adjacencies be formed when this ether3 is not in the correct area?

When you have multiarea ospf, you have to have abr by adding networks in different areas.

You don't. Area is determined by ospf network configuration and instance is determined by area configuration.

That's right, you need to configure the area under the instance that you need; then you can configure the network in the area that you configured before. Finally, need to see if the interface is enabled and if its IP configuration is inside the network configured under the OSPF. If you're using VRFs then you will need to add the routing-table in the instance too.. Once everything is ok, then you'll see the interface with the right area and instance.