One reason why you may not be able to edit your tree online is because the tree was synced from Family Tree Builder, and you are not the original site creator.
If you are the site creator, then with the changes made in FTB 7.0, you can now edit your family tree online at MyHeritage.com. Changes made online will appear on your FTB software after the next sync.
To download Family Tree Builder 7.0, please click here.
As the site creator, you can now give access to your site members to edit your synced tree as well.
To do so, please see here.