When you double click the Art - Target field you'll see various options to choose from. One of the options is for imported assets like your custom model. Choose that, find your model, and hit OK. Now assuming that you haven't messed with anything else, you should see in-game that the buff for Unholy Frenzy now uses your new art.
You can create new copies of anything in the Object Editor, which is what most people do when creating a custom Unit/Ability/Item, etc... So if you wanted to have both the original Unholy Frenzy ability in addition to a new version you would copy and paste the original ability and it's buff and then edit these new copies. You would then link the new buff to the new ability by editing the new abilities Buff field.
You'll see when creating a new copy of something that you get asked about a Rawcode. This can be ignored as the Editor will come up with a unique code that works for you, but do note it's importance. The Rawcode is the ID of the object and no two objects can ever have the same ID. The Rawcode can define important behavior, for instance, giving a Unit a Rawcode that starts with a capital letter will make the Unit a Hero. It may also be referenced in code, which some systems written in Jass/Lua will use.
