Cut on axis or Rotate ACS

HELLO

Im trying to cut a section on the point cloud with quick section / orientated block by axis. It just wont work it stays on the standard x, y.

Can anyone talk me through the few steps? not sure why it wont work it should be really easy.

To get around this i was then trying to Rotate ACS. Cant get this to work either or find any tutorials for either action.

When i select Rotate ACS the first command it gives me is, select view. No idea what this is asking me to do. i have a view selected if i click it on off i just lose the command. Can anyone talk me through this?

I know both are simple just no idea how to start them. Below is a image of the point cloud and the red line is the axis i want to cut the section on. 

Thanks in advance for any help