It normally fails when there is an edge which is not a quad, so either a triangle, or an edge with an added point. Without looking at the blend file, it’s difficult to say. but go into wireframe mode and see what it looks like. Select loop only works until it encounters a non quad face.
Ahh, I am using a PC so not sure how much help I can be. Using the Left ALT key work fine. I wonder, does any addon take over the key for something?
Look in User preferences, under input, put “loop” in the name, it should show Mesh->Loop Select with “Alt Select Mouse” as the action.
If you do, it may be worth trying to assign it to another action. Also, check other commands dependant upon the “alt” key, do any of them work as expected? (Such as Altr+S for shrink/fatten).
Actually, let me try and reproduce the scene, think I know what your are doing.
Edit: Hmm, nope, works fine, can you put up a simple blend file with just that geometry in so I can see if it is OS or blend file related? If your blend file works fine on my system, we know it’s something else.