I'm new to the engine, and I've been having troubles with applying materials to my landscape because I cannot find the landscape details on the interface where i'm supposed to assign. Can someone help me out?

It looks like you closed the details panel. to open a new details panel you should be able to go to the window tab (top left) and in the drop down list you should see details, just select one of the details ones (theres like five i think but they are just multiples of the same thing). last dock the details panel you opened wherever you like by dragging it.