"Open Type" command does not respond in Eclipse workspace

I was already in the right perspective so that was not the cause of my issue. However, closing and re-opening the project fixed the problem for me.

Do you have the right perspective opened? "Open Type" does work in the Java, JavaEE ... perspectives, but not in the "Synchronize" perspective (for example).

To enable it for the "Synchronize" perspective in eclipse Mars(4.5) first close all your opened editors then go to Window > Perspective > Customize Perspective... and in the "Action Set Availability" tab check the "Java Navigation" action set.