I am trying to get the Look At verb to display on my object but I also need to have a custom Verb list in the Verbs tab however the default "Look at" object description mode doesn't appear at all.

I have "Only display verbs from this object's Verbs tab" switched on so do I also need to add a custom Look at verb and use it from the Verbs tab list instead?

If you tick "Only display verbs from this object's Verbs tab", it will only display verbs that are on the object's verbs tab.

If you want to display "Look at" as well, don't tick that. Just edit the "displayverbs" and "inventoryverbs" lists for the object to only include things that you want to show.