On X
- open two files to compare in two separate windows
- M-x ediff-buffers RET RET RET
- now the cursor is in a new small
ediffframe - C-h k w d Now I see a help window explaining that w d runs ediff-buffers and how it works.
Maybe you had the cursor in the main frame instead of the small ediff frame?
On terminal
Same as above, but instead of a new small frame I have a new small (one-line) ediff window. With the cursor on the new window, I can ask for help on w d.
Maybe you had the cursor in one of the two windows containing the files instead of the small ediff window?