In the Proof Central of Elsevier, I don't know how to insert a new inline equation in text.
I read the help but it is helpless:
"Inline equations are handled in the exact same way, however, if you want to place a comment for an inline equation this can’t be triggered through the math editor. Instead you can click on the Comment button in the article view, allowing you to place the comment in the line where the inline equation is mentioned."