The project media folder and project file must reside in the same folder and their names shouldn't be changed. Otherwise, Avid Cinema may not recognize them as being part of the same project.
If the names need to be changed, the media folder should have the same name as the project file with the string " Media" after it (note the space character). For example, if the project file is called "Board Meeting", the project media folder needs to be called "Board Meeting Media".