There was another post on this same question in Support a few weeks ago. Sadly I can't remember how to make a link, but I can tell you it was started by Liminfan(?) on Feb 2 and was called Another Fitday Question... I think. In essence, IIRC the consensus was you need to find an activity that approximates the elliptical in effort and calorie expenditure and use that in your log.
Let me see if I can figure out how to make a linky...

:
Another Fitday Question...
Took me 6 times to get it right, but there it is -- a clicky!