Opened your project in Revit (2017.1) and then went to export to FBX. In my case it was not greyed out.

It seems you use Revit 2014. You can have a look at the following:
-Make sure that you have the latest updates (Revit) installed.
-Make sure that the material libraries are installed properly. Normally they are in C:\Program Files (x86)\Common Files\Autodesk Shared\Materials\Textures\1
-Check your Revit ini file if it is also pointing to this library. It should mention something like:
AdditionalRenderAppearancePaths=..\..\..\Program Files (x86)\Common Files\Autodesk Shared\Materials\Textures\1\Mats
Revit ini normally in C:\Users\username\AppData\Roaming\Autodesk\Revit\Autodesk Revit 201x
Louis

Please mention Revit version, especially when uploading Revit files.