In order to take advantage of the partnership between FamilySearch.org and MyHeritage.com, you are requested to login onto FamilySearch.org with your e-mail and password registration as an LDS Church member.
There you will find a page specifically created to make the connection between your FamilySearch account and MyHeritage account.
If you have further questions on how to find that page in their site, please contact FamilySearch.org.