Hi Neil,
Don’t look too closely at the UI, focus more on the method (which is still very much the same).
You might be struggling with the copy and mirror pasting the poses, which no longer has a button in the UI but rather a hotkey.
To copy a pose, select the control in the viewport, then copy it with Control C.
Go to another frame (if necessary) then mirror paste it with Control Shift V
A normal paste is just Control V.
Bear in mind the mouse must be in the viewport for this to work. If it’s in another window it will copy and paste different data.
Hope that helps.