[czech12]

You need to go into your Firewall Object's Topology page and make sure that the 10.0.50.1 interface is defined in the topology. If it is not, you should be able to click on "Get... > Interfaces" ("Get... > Interfaces with topology" will wipe out any anti-spoofing config you currently have, if any) and that will query the firewall for it's interface names and IP addresses.

Once you have each of the firewall's interfaces listed in the Topology section, you can configure anti-spoofing for each of the interfaces by double clicking on the interface you want to edit. If the only network behind the 10.0.50.1 interface is the 10.0.50.x network, then you can just select "Network Defined by the interface IP and subnet mask", if there are more networks behind that interface, you will need to create a group with those network objects in it, then select "Specific" in the Topology section and select the group you created.

Hope this helps...