I did some additional research on this for you. It appears to be a currently limitation of the software. I apologize for the inconvenience. Even sending an attachment through the operating system may require some manipulation of the Mac OS as you can see in this forum post:
You can see a workaround that was provided here:
You can see additional steps that others have tried here:
http://macmost.com/forum-set-outlook-as-default-mail-client.html
You can certainly try some of the suggestions posted to see if they work for you.
I will be updating the existing request to support using Outlook as the default mail client. You can also share any suggestions here:
http://community.thomsonreuters.com/t5/EndNote-Product-Suggestions/bd-p/en-suggest