Multiple lines of text field in SharePoint 2013 is little different than in 2010. When you are creating one, there are only 2 text type options available.
Here is the observation i made while i am trying to edit multiple lines of text column in edit view
You can edit multiple lines of text column in edit view (Datasheet) only if you select either plain text or Enhanced rich text as text type.
If you choose rich text (refer to this post on how to create multiple lines of text with rich text type), that column is disabled. Microsoft says that it was by design
